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/sock/send.c

@@ -16,12 +16,13 @@
 │ TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR             │
 │ PERFORMANCE OF THIS SOFTWARE.                                                │
 ╚─────────────────────────────────────────────────────────────────────────────*/
+#include "libc/calls/cp.internal.h"
 #include "libc/calls/internal.h"
-#include "libc/intrin/strace.internal.h"
 #include "libc/calls/struct/iovec.h"
 #include "libc/calls/struct/iovec.internal.h"
 #include "libc/dce.h"
 #include "libc/intrin/asan.internal.h"
+#include "libc/intrin/strace.internal.h"
 #include "libc/macros.internal.h"
 #include "libc/sock/internal.h"
 #include "libc/sock/sock.h"
@@ -37,11 +38,14 @@
  * @return number of bytes transmitted, or -1 w/ errno
  * @error EINTR, EHOSTUNREACH, ECONNRESET (UDP ICMP Port Unreachable),
  *     EPIPE (if MSG_NOSIGNAL), EMSGSIZE, ENOTSOCK, EFAULT, etc.
+ * @cancellationpoint
  * @asyncsignalsafe
  * @restartable (unless SO_RCVTIMEO)
  */
 ssize_t send(int fd, const void *buf, size_t size, int flags) {
   ssize_t rc;
+  BEGIN_CANCELLATION_POINT;
+
   if (IsAsan() && !__asan_is_valid(buf, size)) {
     rc = efault();
   } else if (!IsWindows()) {
@@ -61,6 +65,8 @@ ssize_t send(int fd, const void *buf, size_t size, int flags) {
   } else {
     rc = ebadf();
   }
+
+  END_CANCELLATION_POINT;
   DATATRACE("send(%d, %#.*hhs%s, %'zu, %#x) → %'ld% lm", fd,
             MAX(0, MIN(40, rc)), buf, rc > 40 ? "..." : "", size, flags, rc);
   return rc;