bug: `dup2_syscall` validates `oldfd` after the equality fast path

Start in src/rawposix/src/fs_calls.rs at dup2_syscall around line 3154, then inspect process_tests/deterministic/conc_004_dup_close_fork_refcounts.c and skip_test_cases.txt. Run the referenced CONC-004 test to reproduce the invalid equal-descriptor case. Done means invalid oldfd values return EBADF without panicking, valid equal descriptors still succeed, and the test is removed from the skip list.

Component: src/rawposix/src/fs_calls.rs:3154 (dup2_syscall)
Severity: POSIX conformance
Found by: CONC-004 (#1304), conc_004_dup_close_fork_refcounts.c:708

Symptom

dup2(badfd, badfd) reports success and returns an fd number that was never open.
The caller then treats a closed descriptor as valid.

Root cause

// src/rawposix/src/fs_calls.rs:3154
if old_vfd_arg > MAXFD as u64 || new_vfd_arg > MAXFD as u64 {
    return syscall_error(Errno::EBADF, "dup2", "Bad File Descriptor");
} else if old_vfd_arg == new_vfd_arg {
    // Does nothing
    return new_vfd_arg as i32;      // <-- oldfd never validated
}

POSIX: "If oldfd is not a valid file descriptor, then the call fails, and newfd is
not closed." dup2() returns newfd unchanged only when oldfd is valid and
equal to it. The range check catches out-of-range numbers but not in-range-but-closed
ones.

There is a second defect in the same function: the get_specific_virtual_fd(...)
call on the success path ends in .unwrap(), panicking the host on failure.

Proposed fix

Move validation before the equality check, and replace the .unwrap() with EBADF:

     if old_vfd_arg > MAXFD as u64 || new_vfd_arg > MAXFD as u64 {
         return syscall_error(Errno::EBADF, "dup2", "Bad File Descriptor");
-    } else if old_vfd_arg == new_vfd_arg {
-        // Does nothing
+    }
+
+    // `oldfd` must be validated BEFORE the `oldfd == newfd` fast path.
+    let old_vfd = match fdtables::translate_virtual_fd(cageid, old_vfd_arg) {
+        Ok(entry) => entry,
+        Err(_e) => return syscall_error(Errno::EBADF, "dup2", "Bad File Descriptor"),
+    };
+
+    if old_vfd_arg == new_vfd_arg {
+        // oldfd is valid and equals newfd: no-op, newfd is NOT closed.
         return new_vfd_arg as i32;
     }

then the existing body, with .unwrap() replaced by a match returning EBADF.

Guest-visible change: dup2(badfd, badfd) now fails with EBADF; previously it
"succeeded".

Un-skip on merge

Remove process_tests/deterministic/conc_004_dup_close_fork_refcounts.c from
skip_test_cases.txt.