fork: add redox_fcntl_v0 + redox_rt::sys::fcntl for redox-scheme 0.11.2

Imported from upstream relibc master: redox_fcntl_v0 in src/platform/redox/libredox.rs
and a thin redox_rt::sys::fcntl(fd, cmd, arg) wrapper around syscall::fcntl.

redox-scheme 0.11.2 from crates.io references redox_fcntl_v0 directly (it is
declared as an extern in libredox 0.1.18 and our local fork has it). Without
this symbol, every user daemon that uses redox-scheme 0.11.2 fails to link
with: undefined reference to redox_fcntl_v0.

This is the kind of upstream-tracking gap that the AGENTS.md fork-sync
policy exists to close. Upstream already had the function; we just had not
replayed that part of upstream history into the fork yet.

Bump the relibc fork forward in functional terms; no Cargo.toml version
change (per AGENTS.md fork-sync policy, the upstream-tag part stays
'0.2.5' and the rb-counter resets on a real upstream bump, not on
function additions).

redox-rt/src/sys.rs

@@ -561,3 +561,8 @@ pub fn std_fs_call_rw(fd: usize, payload: &mut [u8], metadata: &StdFsCallMeta) -
 pub fn fstat(fd: usize, stat: &mut syscall::Stat) -> Result<usize> {
     std_fs_call_ro(fd, stat, &StdFsCallMeta::new(StdFsCallKind::Fstat, 0, 0))
 }
+
+pub fn fcntl(fd: usize, cmd: usize, arg: usize) -> Result<usize> {
+    let _siglock = tmp_disable_signals();
+    FdGuard::new(syscall::fcntl(fd, cmd, arg)).map(|g| g.as_raw_fd())
+}

src/platform/redox/libredox.rs

@@ -378,6 +378,11 @@ pub unsafe extern "C" fn redox_close_v1(fd: usize) -> RawResult {
     Error::mux(syscall::close(fd))
 }
 
+#[unsafe(no_mangle)]
+pub unsafe extern "C" fn redox_fcntl_v0(fd: usize, cmd: usize, arg: usize) -> RawResult {
+    Error::mux(redox_rt::sys::fcntl(fd, cmd, arg))
+}
+
 #[unsafe(no_mangle)]
 pub unsafe extern "C" fn redox_get_pid_v1() -> RawResult {
     redox_rt::sys::posix_getpid() as _