Job control, function keys, and real sleeps

- SIGSTOP/SIGTSTP/SIGTTIN/SIGTTOU stop, SIGCONT resumes; the parent gets
  SIGCHLD and wait4 reports WUNTRACED/WCONTINUED; ps shows T. Ctrl-Z works,
  and mc no longer hangs waiting for its subshell to stop.
- Blocking reads (console, pty, pipe) and sleeps take a pending stop and
  carry on after SIGCONT; a caught signal without SA_RESTART ends them
  with EINTR.
- nanosleep reads tv_nsec (sleep 0.5 used to return at once), writes the
  remainder, and clock_nanosleep understands TIMER_ABSTIME.
- rt_sigaction uses the k_sigaction layout for Linux-ABI processes: the old
  action's restorer was written 112 bytes past the caller's struct. Flags
  are kept; SIGKILL and SIGSTOP cannot be caught.
- F1..F12 reach programs, as the Linux console sends them (PS/2, USB, I2C).
- TIOCLINUX subcode 6 answers "no modifiers" instead of echoing the 6 back.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01S1giNvrHTrPMPw7V6x1kQ4

kernel/sys/sys_fs.c

@@ -1653,6 +1653,20 @@ addr_t sys_ioctl(void)
     // terminal and nothing to choose. It records the choice now, which is what
     // /dev/tty resolves through and what a session leader on tty3 needs so
     // that its /dev/tty is tty3 and not whatever is on screen.
+    // TIOCLINUX. Only subcode 6, the modifier state, is answered -- with
+    // "none held", which is true often enough. mc asks it after every key on
+    // a Linux console, and the ioctl used to succeed without writing: the 6
+    // it passed in came back as Ctrl+AltGr held, and Enter arrived as an
+    // unknown chord printed as a dot.
+    if (request == 0x541C) {
+        uchar sub;
+        if (copy_from_user(&sub, argp, 1) < 0)
+            return -EFAULT;
+        if (sub != 6)
+            return -EINVAL;
+        sub = 0;
+        return copy_to_user(argp, &sub, 1) < 0 ? -EFAULT : 0;
+    }
     if (request == TIOCSCTTY) {
         struct vt *v = vt_for_minor(f->ip->minor);
         myproc()->ctty = vt_index(v) + 1;