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/connect.c

@@ -16,6 +16,7 @@
 │ 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/dce.h"
 #include "libc/intrin/asan.internal.h"
@@ -41,6 +42,8 @@
  */
 int connect(int fd, const struct sockaddr *addr, uint32_t addrsize) {
   int rc;
+  BEGIN_CANCELLATION_POINT;
+
   if (addr && !(IsAsan() && !__asan_is_valid(addr, addrsize))) {
     if (!IsWindows()) {
       rc = sys_connect(fd, addr, addrsize);
@@ -52,6 +55,8 @@ int connect(int fd, const struct sockaddr *addr, uint32_t addrsize) {
   } else {
     rc = efault();
   }
+
+  END_CANCELLATION_POINT;
   STRACE("connect(%d, %s) → %d% lm", fd, DescribeSockaddr(addr, addrsize), rc);
   return rc;
 }