Improve cancellations, randomness, and time

- Exhaustively document cancellation points
- Rename SIGCANCEL to SIGTHR just like BSDs
- Further improve POSIX thread cancellations
- Ensure asynchronous cancellations work correctly
- Elevate the quality of getrandom() and getentropy()
- Make futexes cancel correctly on OpenBSD 6.x and 7.x
- Add reboot.com and shutdown.com to examples directory
- Remove underscore prefix from awesome timespec_*() APIs
- Create assertions that help verify our cancellation points
- Remove bad timespec APIs (cmp generalizes eq/ne/gt/gte/lt/lte)

examples/reboot.c

@@ -0,0 +1,39 @@
+#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/runtime/runtime.h"
+#include "libc/stdio/stdio.h"
+#include "libc/str/str.h"
+#include "libc/sysv/consts/reboot.h"
+
+int main(int argc, char *argv[]) {
+  char line[8] = {0};
+  if (argc > 1 && !strcmp(argv[1], "-y")) {
+    line[0] = 'y';
+  } else {
+    printf("reboot your computer? yes or no [no] ");
+    fflush(stdout);
+    fgets(line, sizeof(line), stdin);
+  }
+  if (line[0] == 'y' || line[0] == 'Y') {
+    if (reboot(RB_AUTOBOOT)) {
+      printf("system is rebooting...\n");
+      exit(0);
+    } else {
+      perror("reboot");
+      exit(1);
+    }
+  } else if (line[0] == 'n' || line[0] == 'N') {
+    exit(0);
+  } else {
+    printf("error: unrecognized response\n");
+    exit(2);
+  }
+}