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)

libc/calls/execve-sysv.c

@@ -16,6 +16,7 @@
 │ TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR             │
 │ PERFORMANCE OF THIS SOFTWARE.                                                │
 ╚─────────────────────────────────────────────────────────────────────────────*/
+#include "libc/calls/blockcancel.internal.h"
 #include "libc/calls/blocksigs.internal.h"
 #include "libc/calls/calls.h"
 #include "libc/calls/syscall-sysv.internal.h"
@@ -40,6 +41,8 @@ static bool IsApeBinary(const char *path) {
   int fd;
   char buf[8];
   bool res = false;
+  // TODO(jart): Should we block signals too?
+  BLOCK_CANCELLATIONS;
   if ((fd = sys_open(path, O_RDONLY, 0)) != -1) {
     if (sys_read(fd, buf, 8) == 8 &&  //
         (READ64LE(buf) == READ64LE("MZqFpD='") ||
@@ -48,6 +51,7 @@ static bool IsApeBinary(const char *path) {
     }
     sys_close(fd);
   }
+  ALLOW_CANCELLATIONS;
   return res;
 }