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/drivers/usb/hid/usb_hid.c

@@ -168,6 +168,8 @@ hid_special(uchar usage, uchar mods)
   if (usage >= 0x3A && usage < 0x3A + NVT && (mods & HID_MOD_ALT) &&
       !(mods & HID_MOD_SHIFT))
     return KBD_VT_SWITCH + (usage - 0x3A);
+  if (usage >= 0x3A && usage <= 0x45)
+    return KBD_F1 + (usage - 0x3A);
   switch (usage) {
   case 0x49: return KBD_INS;
   case 0x4A: return KBD_HOME;