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/pwrite.c

@@ -18,6 +18,7 @@
 ╚─────────────────────────────────────────────────────────────────────────────*/
 #include "libc/assert.h"
 #include "libc/calls/calls.h"
+#include "libc/calls/cp.internal.h"
 #include "libc/calls/internal.h"
 #include "libc/calls/struct/iovec.h"
 #include "libc/calls/struct/iovec.internal.h"
@@ -50,6 +51,8 @@
 ssize_t pwrite(int fd, const void *buf, size_t size, int64_t offset) {
   ssize_t rc;
   size_t wrote;
+  BEGIN_CANCELLATION_POINT;
+
   if (offset < 0) {
     rc = einval();
   } else if (fd == -1) {
@@ -71,6 +74,8 @@ ssize_t pwrite(int fd, const void *buf, size_t size, int64_t offset) {
       _npassert(wrote <= size);
     }
   }
+
+  END_CANCELLATION_POINT;
   DATATRACE("pwrite(%d, %#.*hhs%s, %'zu, %'zd) → %'zd% m", fd,
             MAX(0, MIN(40, rc)), buf, rc > 40 ? "..." : "", size, offset, rc);
   return rc;