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/read-nt.c

@@ -51,11 +51,7 @@ textwindows ssize_t sys_read_nt(struct Fd *fd, const struct iovec *iov,
   ssize_t rc;
   uint32_t size;
   size_t i, total;
-  if (cmpxchg(&__interrupted, true, false) ||
-      (weaken(_check_sigchld) && weaken(_check_sigchld)()) ||
-      (weaken(_check_sigwinch) && weaken(_check_sigwinch)(fd))) {
-    return eintr();
-  }
+  if (_check_interrupts(true, fd)) return eintr();
   while (iovlen && !iov[0].iov_len) iov++, iovlen--;
   if (iovlen) {
     for (total = i = 0; i < iovlen; ++i) {