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/msync.c

@@ -18,6 +18,7 @@
 ╚─────────────────────────────────────────────────────────────────────────────*/
 #include "libc/assert.h"
 #include "libc/calls/calls.h"
+#include "libc/calls/cp.internal.h"
 #include "libc/calls/syscall-nt.internal.h"
 #include "libc/calls/syscall-sysv.internal.h"
 #include "libc/dce.h"
@@ -34,16 +35,22 @@
  * @param addr needs to be 4096-byte page aligned
  * @param flags needs MS_ASYNC or MS_SYNC and can have MS_INVALIDATE
  * @return 0 on success or -1 w/ errno
+ * @raise ECANCELED if thread was cancelled in masked mode
+ * @raise EINTR if we needed to block and a signal was delivered instead
  * @cancellationpoint
  */
 int msync(void *addr, size_t size, int flags) {
   int rc;
+  BEGIN_CANCELLATION_POINT;
+
   _unassert(((flags & MS_SYNC) ^ (flags & MS_ASYNC)) || !(MS_SYNC && MS_ASYNC));
   if (!IsWindows()) {
     rc = sys_msync(addr, size, flags);
   } else {
     rc = sys_msync_nt(addr, size, flags);
   }
+
+  END_CANCELLATION_POINT;
   STRACE("msync(%p, %'zu, %#x) → %d% m", addr, size, flags, rc);
   return rc;
 }