Make improvements

- This change fixes a bug that allowed unbuffered printf() output (to
  streams like stderr) to be truncated. This regression was introduced
  some time between now and the last release.

- POSIX specifies all functions as thread safe by default. This change
  works towards cleaning up our use of the @threadsafe / @threadunsafe
  documentation annotations to reflect that. The goal is (1) to use
  @threadunsafe to document functions which POSIX say needn't be thread
  safe, and (2) use @threadsafe to document functions that we chose to
  implement as thread safe even though POSIX didn't mandate it.

- Tidy up the clock_gettime() implementation. We're now trying out a
  cleaner approach to system call support that aims to maintain the
  Linux errno convention as long as possible. This also fixes bugs that
  existed previously, where the vDSO errno wasn't being translated
  properly. The gettimeofday() system call is now a wrapper for
  clock_gettime(), which reduces bloat in apps that use both.

- The recently-introduced improvements to the execute bit on Windows has
  had bugs fixed. access(X_OK) on a directory on Windows now succeeds.
  fstat() will now perform the MZ/#! ReadFile() operation correctly.

- Windows.h is no longer included in libc/isystem/, because it confused
  PCRE's build system into thinking Cosmopolitan is a WIN32 platform.
  Cosmo's Windows.h polyfill was never even really that good, since it
  only defines a subset of the subset of WIN32 APIs that Cosmo defines.

- The setlongerjmp() / longerjmp() APIs are removed. While they're nice
  APIs that are superior to the standardized setjmp / longjmp functions,
  they weren't superior enough to not be dead code in the monorepo. If
  you use these APIs, please file an issue and they'll be restored.

- The .com appending magic has now been removed from APE Loader.

libc/sysv/consts.sh

@@ -584,7 +584,7 @@ syscon	clock	CLOCK_MONOTONIC_PRECISE			1			1			1			6			11			3			3			1			#
 syscon	clock	CLOCK_MONOTONIC_FAST			1			1			1			6			12			3			3			1			#
 syscon	clock	CLOCK_MONOTONIC_COARSE			6			6			1			6			12			3			3			1			# Linux 2.6.32+; bsd consensus; not available on RHEL5
 syscon	clock	CLOCK_MONOTONIC_RAW			4			4			127			4			127			127			127			127			# actually monotonic; not subject to NTP adjustments; Linux 2.6.28+; XNU/NT/FreeBSD/OpenBSD faked; not available on RHEL5
-syscon	clock	CLOCK_PROCESS_CPUTIME_ID		2			2			127			12			15			2			0x40000000		127			#
+syscon	clock	CLOCK_PROCESS_CPUTIME_ID		2			2			127			12			15			2			0x40000000		127			# NetBSD lets you bitwise a PID into clockid_t
 syscon	clock	CLOCK_THREAD_CPUTIME_ID			3			3			127			16			14			4			0x20000000		127			#
 syscon	clock	CLOCK_PROF				127			127			127			127			2			127			2			127			#
 syscon	clock	CLOCK_BOOTTIME				7			7			127			127			127			6			127			127			#