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/setitimer.c

@@ -0,0 +1,72 @@
+#if 0
+/*─────────────────────────────────────────────────────────────────╗
+│ To the extent possible under law, Justine Tunney has waived      │
+│ all copyright and related or neighboring rights to this file,    │
+│ as it is written in the following disclaimers:                   │
+│   • http://unlicense.org/                                        │
+│   • http://creativecommons.org/publicdomain/zero/1.0/            │
+╚─────────────────────────────────────────────────────────────────*/
+#endif
+#include "libc/calls/calls.h"
+#include "libc/calls/struct/itimerval.h"
+#include "libc/calls/struct/sigaction.h"
+#include "libc/calls/struct/siginfo.h"
+#include "libc/calls/ucontext.h"
+#include "libc/log/check.h"
+#include "libc/stdio/stdio.h"
+#include "libc/sysv/consts/itimer.h"
+#include "libc/sysv/consts/sa.h"
+#include "libc/sysv/consts/sig.h"
+#include "libc/time/time.h"
+
+volatile bool gotalrm;
+
+void OnSigAlrm(int sig, siginfo_t *si, ucontext_t *ctx) {
+  gotalrm = true;
+}
+
+int main() {
+
+  //////////////////////////////////////////////////////////////////////////////
+  printf("first tutorial: set singleshot alarm for 1.5 seconds from now\n");
+
+  // setup alarm in 1.5 seconds
+  // this timer tears itself down once it's handled
+  struct itimerval shot = {{0, 0}, {1, 500000}};
+  struct sigaction handler = {.sa_sigaction = OnSigAlrm,
+                              .sa_flags = SA_RESETHAND | SA_SIGINFO};
+  CHECK_EQ(0, sigaction(SIGALRM, &handler, 0));
+  CHECK_EQ(0, setitimer(ITIMER_REAL, &shot, 0));
+
+  // wait for alarm
+  pause();
+  printf("got singleshot alarm!\n\n");
+
+  //////////////////////////////////////////////////////////////////////////////
+  printf("second tutorial: interval timer\n");
+
+  // setup timer every 1.5 seconds
+  struct sigaction oldalrm;
+  struct sigaction sigalrm = {.sa_sigaction = OnSigAlrm,
+                              .sa_flags = SA_SIGINFO};
+  CHECK_EQ(0, sigaction(SIGALRM, &sigalrm, &oldalrm));
+  struct itimerval oldtimer;
+  struct itimerval timer = {{1, 500000}, {1, 500000}};
+  CHECK_EQ(0, setitimer(ITIMER_REAL, &timer, &oldtimer));
+
+  // wait for three timeouts
+  int i = 0;
+  int n = 3;
+  while (i < n) {
+    pause();
+    if (gotalrm) {
+      ++i;
+      printf("got timeout %d out of %d\n", i, n);
+      gotalrm = false;
+    }
+  }
+
+  // teardown timer
+  CHECK_EQ(0, setitimer(ITIMER_REAL, &oldtimer, 0));
+  CHECK_EQ(0, sigaction(SIGALRM, &oldalrm, 0));
+}