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/dev/pty.c

@@ -458,6 +458,16 @@ pts_read(struct inode *ip, uint64 off, char *dst, int n)
         ilock(ip);
         return got ? got : -EINTR;
       }
+      if (sig_attention(myproc())) {
+        release(&ptylock);
+        int e = sig_blocking_check();
+        if (e) {
+          ilock(ip);
+          return got ? got : e;
+        }
+        acquire(&ptylock);
+        continue;
+      }
       sleep(&p->ir, &ptylock);
     }
     c = p->in[p->ir++ % PTY_BUF];