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

examples/exec.c

@@ -8,14 +8,24 @@
 ╚─────────────────────────────────────────────────────────────────*/
 #endif
 #include "libc/calls/calls.h"
+#include "libc/calls/sigbits.h"
+#include "libc/calls/struct/sigset.h"
 #include "libc/runtime/runtime.h"
 #include "libc/stdio/stdio.h"
+#include "libc/sysv/consts/sig.h"
 
 int main(int argc, char *argv[]) {
+  sigset_t ss;
   if (argc < 3) {
     fputs("USAGE: EXEC.COM PROG ARGV₀ [ARGV₁...]\n", stderr);
     return 1;
   }
+
+  // block arbitrary signal so __printargs() looks cooler
+  sigemptyset(&ss);
+  sigaddset(&ss, SIGPWR);
+  sigprocmask(SIG_BLOCK, &ss, 0);
+
   execv(argv[1], argv + 1);
   return 127;
 }