bootstrap: fix FD allocation to match upstream userspace fd model

The bootstrap's start.rs used libredox::call::openat which resolved to
the bootstrap's own redox_openat_v1 in initfs.rs, calling syscall::openat
(SYS_OPENAT). This let the kernel allocate POSIX FDs 0,1,2 for
stdin/stdout/stderr, but the userspace FILETABLE was never updated.

Later, exec.rs called auth.dup(b'cur-context') via redox_rt::sys::dup,
which asked the empty FILETABLE for a free slot (returning 0), then
passed it to SYS_DUP_INTO. The kernel rejected it with EEXIST because
posix_fdtbl[0] was already occupied by stdin.

Fix: use syscall::openat_into to directly specify FD slots 0,1,2 in
start.rs, and use syscall::dup_into with a computed FD index for
cur-context in exec.rs. Both bypass the FILETABLE, which is only
populated later by redox_rt::initialize_freestanding.

bootstrap/src/exec.rs

@@ -53,6 +53,8 @@ pub fn main() -> ! {
         FdGuard::new(*(base_ptr as *const usize))
     };
 
+    let cur_context_idx = scheme_creation_cap.as_raw_fd() + 1;
+
     let mut kernel_schemes = KernelSchemeMap::new(kernel_scheme_infos);
 
     let auth = kernel_schemes
@@ -60,8 +62,8 @@ pub fn main() -> ! {
         .remove(&GlobalSchemes::Proc)
         .expect("failed to get proc fd");
 
-    let this_thr_fd = auth
-        .dup(b"cur-context")
+    let this_thr_fd = syscall::dup_into(auth.as_raw_fd(), cur_context_idx, b"cur-context")
+        .map(FdGuard::new)
         .expect("failed to open open_via_dup")
         .to_upper()
         .unwrap();