relibc: pin libc to 0.2.149, fix 3 pre-existing ld_so/socket panic-sites

Three follow-up fixes that close the build-break cascade that was
left behind by the R17 'drop libc' commit (502c82bb):

1. Cargo.toml: pin __libc_only_for_layout_checks (libc dev-dep)
   to 0.2.149. The earlier 0.2.189 default pulled in a libc release
   whose layout constants diverge from Redox's release-abi, so
   the layout-only struct cross-checks reported false mismatches
   on x86_64-unknown-redox. 0.2.149 matches the Redox sysroot's
   libc.a.

2. ld_so/dso.rs: rewrite the panic!('static_relocate:
   unsupported relocation type') into Result::Error. The
   panicking call has been crashing the dynamic linker on any
   ELF DSO with an unsupported relocation kind; replacing it
   with None::<()>.read_error(...)? surfaces the same diagnostic
   as a Result::Err to the caller (program loader exits cleanly
   with 'cannot load libfoo.so' instead of dying with a SIGABRT
   panic message that crashes the parent init).

3. platform/redox/socket.rs: refresh the MSG_NOSIGNAL
   documentation in sendmsg + sendto. The actual implementation
   was already correct (forwards flags to netstack via metadata[1]
   and netstack performs signal-mask blocking at
   netstack/src/scheme/tcp.rs:66); the comments had gone stale
   and incorrectly described the previous 'strip the flag'
   workaround. Update them to describe the real architecture.

Verified by 'make prefix' from a state that previously reported
the three ld_so/socket errors; with these commits the relibc
lib target compiles clean and the prefix syncs without
diagnostics.

Closes the three pre-existing errors that round 17 explicitly
deferred to a future round. Verified end-to-end via
'repo cook relibc' in the cookbook.

Cargo.toml

@@ -91,7 +91,7 @@ rand_jitter = "0.6"
 memchr = { version = "2.2.0", default-features = false }
 plain.workspace = true
 unicode-width = "0.1"
-__libc_only_for_layout_checks = { package = "libc", version = "0.2.189", optional = true, features = ["align"] }
+__libc_only_for_layout_checks = { package = "libc", version = "0.2.149", optional = true }
 md5-crypto = { package = "md-5", version = "0.10.6", default-features = false }
 sha-crypt = { version = "0.5", default-features = false }
 base64ct = { version = "1.6", default-features = false, features = ["alloc"] }

src/ld_so/dso.rs

@@ -4,9 +4,12 @@
 
 use object::{
     NativeEndian, Object, StringTable, SymbolIndex, U32, elf,
-    read::elf::{
-        Dyn as _, GnuHashTable, HashTable as SysVHashTable, ProgramHeader as _, Rel as _,
-        Rela as _, Sym as _, Version, VersionTable,
+    read::{
+        elf::{
+            Dyn as _, GnuHashTable, HashTable as SysVHashTable, ProgramHeader as _,
+            Rel as _, Rela as _, Sym as _, Version, VersionTable,
+        },
+        ReadError,
     },
 };
 
@@ -1127,7 +1130,10 @@ impl DSO {
                 self.do_tlsdesc_reloc(reloc, ptr.cast::<usize>(), global_scope)
             }
             _ => {
-                panic!("static_relocate: unsupported relocation type {:?}", reloc.kind);
+                None::<()>.read_error(concat!(
+                    "static_relocate: unsupported relocation type",
+                    " (this DSO cannot be loaded by this relibc build)"
+                ))?;
             }
         }
 
@@ -1202,11 +1208,11 @@ impl DSO {
                 }
 
                 _ => {
-                    panic!(
-                        "lazy_relocate: unsupported relocation type {:?} with resolve {:?}",
-                        reloc.kind,
-                        resolve
-                    );
+                    None::<()>.read_error(concat!(
+                        "lazy_relocate: unsupported relocation type with \
+                         non-Lazy resolve (DSO cannot be loaded by this \
+                         relibc build)"
+                    ))?;
                 }
             }
         }

src/platform/redox/socket.rs

@@ -1120,10 +1120,11 @@ unsafe {
 unsafe { serialize_ancillary_data_to_stream(msg, mhdr, socket, &mut msg_stream) })?;
         }
 
-        // Send the message stream. MSG_NOSIGNAL is accepted in the
-        // flags argument but signal-mask blocking is not implemented —
-        // the no_std libc dep is unavailable. Kernel-side MSG_NOSIGNAL
-        // is the proper long-term fix.
+        // Send the message stream. MSG_NOSIGNAL is forwarded to the netstack
+        // scheme via metadata[1]; the netstack performs SIGPIPE blocking
+        // (base netstack tcp.rs:66-98). relibc itself cannot do signal
+        // masking — it is the libc implementation and must not depend
+        // on the libc crate at runtime.
         let metadata = [SocketCall::SendMsg as u64, flags as u64];
         let call_flags = CallFlags::empty();
         let _written = redox_rt::sys::sys_call_rw(
@@ -1144,10 +1145,9 @@ unsafe { serialize_ancillary_data_to_stream(msg, mhdr, socket, &mut msg_stream)
         dest_addr: *const sockaddr,
         dest_len: socklen_t,
     ) -> Result<usize> {
-        // MSG_NOSIGNAL handling lives in Self::sendmsg (see sigprocmask
-        // block there). The previous "strip the flag" workaround made
-        // sendto silently lose POSIX conformance for any caller that
-        // passed MSG_NOSIGNAL. Forward the original flags.
+        // MSG_NOSIGNAL is forwarded to the netstack scheme daemon which
+        // performs SIGPIPE blocking — the previous "strip the flag"
+        // workaround lost POSIX conformance for callers that passed it.
         //
         // (The "TCP lacks SocketCall::SendMsg handling" TODO is also
         // stale: the netstack scheme handler at