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/log/backtrace2.c

@@ -54,6 +54,13 @@ static int PrintBacktraceUsingAddr2line(int fd, const struct StackFrame *bp) {
   char *debugbin, *p1, *p2, *p3, *addr2line;
   char buf[kBacktraceBufSize], *argv[kBacktraceMaxFrames];
 
+  // DWARF is a weak standard. Platforms that use LLVM or old GNU
+  // usually can't be counted upon to print backtraces correctly.
+  if (!IsLinux() && !IsWindows()) {
+    ShowHint("won't print addr2line backtrace because probably llvm");
+    return -1;
+  }
+
   if (!PLEDGED(STDIO) || !PLEDGED(EXEC) || !PLEDGED(EXEC)) {
     return -1;
   }
@@ -69,13 +76,6 @@ static int PrintBacktraceUsingAddr2line(int fd, const struct StackFrame *bp) {
     return -1;
   }
 
-  // DWARF is a weak standard. Platforms that use LLVM or old GNU
-  // usually can't be counted upon to print backtraces correctly.
-  if (!IsLinux() && !IsWindows()) {
-    ShowHint("won't print addr2line backtrace because probably llvm");
-    return -1;
-  }
-
   // backtrace_test.com failing on windows for some reason via runitd
   // don't want to pull in the high-level syscalls here anyway
   if (IsWindows()) {