Make signal handling work well across platforms

- Fix sigsuspend() on XNU
- Fix strsignal() on non-Linux
- Add unit tests for strsignal()
- Add unit tests for setitimer()
- Add unit tests for sigsuspend()
- Rewrite setitimer() for New Technology
- Rewrite nanosleep() for New Technology
- Polyfill SIGALRM on the New Technology
- select(0,0,0,0) on NT now calls pause()
- Remove some NTDLL calls that aren't needed
- Polyfill SA_NOCLDWAIT on the New Technology
- Polyfill SA_RESETHAND on the New Technology
- Polyfill sigprocmask() on the New Technology
- Polyfill SIGCHLD+SIG_IGN on the New Technology
- Polyfill SA_RESTART masking on the New Technology
- Deliver console signals from main thread on New Technology
- Document SA_RESTART behavior w/ @sarestartable / @norestart
- System call trace in MODE=dbg now prints inherited FDs and signal mask

libc/calls/sigaction.c

@@ -219,6 +219,7 @@ static int __sigaction(int sig, const struct sigaction *act,
     }
     if (act) {
       __sighandrvas[sig] = rva;
+      __sighandflags[sig] = act->sa_flags;
     }
   }
   return rc;
@@ -232,6 +233,7 @@ static int __sigaction(int sig, const struct sigaction *act,
  *                            .sa_flags = SA_RESETHAND|SA_RESTART|SA_SIGINFO};
  *     CHECK_NE(-1, sigaction(SIGINT, &sa, NULL));
  *
+ * @return 0 on success or -1 w/ errno
  * @see xsigaction() for a much better api
  * @asyncsignalsafe
  * @vforksafe