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/fmt/strerror_short.greg.c

@@ -29,7 +29,7 @@ privileged const char *strerror_short(int x) {
   int i;
   if (x) {
     for (i = 0; kErrorNames[i].x; ++i) {
-      if (x == *(const *)((uintptr_t)kErrorNames + kErrorNames[i].x)) {
+      if (x == *(const int *)((uintptr_t)kErrorNames + kErrorNames[i].x)) {
         return (const char *)((uintptr_t)kErrorNames + kErrorNames[i].s);
       }
     }