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/fs/pipe.c

@@ -183,6 +183,14 @@ piperead(struct pipe *p, char *addr, int n)
       release(&p->lock);
       return -1;
     }
+    if(sig_attention(myproc())){     // stopped, or a handler wants to run
+      release(&p->lock);
+      int e = sig_blocking_check();
+      if(e)
+        return e;
+      acquire(&p->lock);
+      continue;
+    }
     sleep(&p->nread, &p->lock); //DOC: piperead-sleep
   }
   i = 0;