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/sysv/consts.sh

@@ -168,10 +168,10 @@ syscon	sig	SIGPROF					27			27			27			27			27			27			# profiling timer expired;
 syscon	sig	SIGWINCH				28			28			28			28			28			28			# terminal resized; unix consensus & faked on nt
 syscon	sig	SIGIO					29			23			23			23			23			29			# bsd consensus
 syscon	sig	SIGSYS					31			12			12			12			12			31			# wut; bsd consensus
-syscon	sig	SIGINFO					0			29			29			29			29			0			# bsd consensus
-syscon	sig	SIGEMT					0			7			7			7			7			0			# not implemented in most community editions of system five; consider doing this using SIGUSR1 or SIGUSR2 instead
+syscon	sig	SIGINFO					63			29			29			29			29			63			# bsd consensus
+syscon	sig	SIGEMT					64			7			7			7			7			64			# not implemented in most community editions of system five; consider doing this using SIGUSR1 or SIGUSR2 instead
 syscon	sig	SIGPWR					30			30			30			30			32			30			# not implemented in most community editions of system five; consider doing this using SIGUSR1 or SIGUSR2 instead
-syscon	sig	SIGCANCEL				32			7			65			7			33			32			# SIGRTMIN+0; faked as SIGEMT on XNU and OpenBSD
+syscon	sig	SIGTHR					32			7			32			32			33			32			# used by pthread_cancel(); SIGRTMIN+0 on Linux/NetBSD; faked as SIGEMT on XNU (what is SIG32 on XNU anyway?)
 syscon	sig	SIGRTMIN				32			0			65			0			33			32
 syscon	sig	SIGRTMAX				64			0			126			0			63			64
 syscon	compat	SIGPOLL					29			23			23			23			23			29			# same as SIGIO