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/testlib/testrunner.c

@@ -225,8 +225,10 @@ void testlib_runtestcases(testfn_t *start, testfn_t *end, testfn_t warmup) {
     if (!IsWindows()) sys_getpid();
     if (warmup) warmup();
     testlib_clearxmmregisters();
-    STRACE("running test %t", fn);
+    STRACE("");
+    STRACE("# running test %t", fn);
     (*fn)();
+    STRACE("");
     if (!IsWindows()) sys_getpid();
     if (_weaken(TearDown)) _weaken(TearDown)();
     if (_weaken(testlib_enable_tmp_setup_teardown)) TearDownTmpDir();