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

include/core/proc.h

@@ -175,6 +175,7 @@ struct procinfo {
   char   name[16];
   char   exe[64];           // full path, for /proc/<pid>/exe
   int    kthread;           // a kernel thread, not a program
+  int    stopped;           // stopped by a signal (ps shows T)
 };
 
 struct proc {
@@ -248,6 +249,7 @@ struct proc {
   // support nested signal delivery).
   uint64 sig_handler[32];
   uint64 sig_restorer[32];
+  uint64 sig_flags[32];             // sa_flags; SA_RESTART is the one read
   struct trapframe saved_sigtf;
   int in_signal;
 
@@ -330,6 +332,13 @@ struct proc {
 
   // umask(2). Inherited across fork; 022 for the first process.
   uint umask;
+
+  // Job control. stopsig is the signal that stopped the process, 0 while it
+  // runs; stop_report and cont_report are what wait(WUNTRACED/WCONTINUED)
+  // has yet to tell the parent.
+  int stopsig;
+  int stop_report;
+  int cont_report;
 };
 #define FD_CLOEXEC 1