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/runtime/ezmap.c

@@ -16,8 +16,8 @@
 │ TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR             │
 │ PERFORMANCE OF THIS SOFTWARE.                                                │
 ╚─────────────────────────────────────────────────────────────────────────────*/
-#include "libc/intrin/safemacros.internal.h"
 #include "libc/calls/calls.h"
+#include "libc/intrin/safemacros.internal.h"
 #include "libc/limits.h"
 #include "libc/log/libfatal.internal.h"
 #include "libc/runtime/ezmap.internal.h"
@@ -26,6 +26,8 @@
 #include "libc/sysv/consts/o.h"
 #include "libc/sysv/consts/prot.h"
 
+// TODO(jart): DELETE
+
 hidden int MapFileRead(const char *filename, struct MappedFile *mf) {
   mf->addr = MAP_FAILED;
   if ((mf->fd = open(filename, O_RDONLY)) != -1 &&