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/rand/getrandom.c

@@ -62,6 +62,7 @@ static bool have_getrandom;
  * close any file descriptor it ends up needing before it returns.
  *
  * @asyncsignalsafe
+ * @restartable
  * @vforksafe
  */
 ssize_t getrandom(void *p, size_t n, unsigned f) {