Make more fixes and improvements

- Remove PAGESIZE constant
- Fix realloc() documentation
- Fix ttyname_r() error reporting
- Make forking more reliable on Windows
- Make execvp() a few microseconds faster
- Make system() a few microseconds faster
- Tighten up the socket-related magic numbers
- Loosen restrictions on mmap() offset alignment
- Improve GetProgramExecutableName() with getenv("_")
- Use mkstemp() as basis for mktemp(), tmpfile(), tmpfd()
- Fix flakes in pthread_cancel_test, unix_test, fork_test
- Fix recently introduced futex stack overflow regression
- Let sockets be passed as stdio to subprocesses on Windows
- Improve security of bind() on Windows w/ SO_EXCLUSIVEADDRUSE

test/libc/intrin/pthread_atfork_test.c

@@ -99,7 +99,6 @@ void *Worker(void *arg) {
 }
 
 TEST(pthread_atfork, torture) {
-  if (IsWindows()) return;  // TODO(jart): why do we get EBADF? it worked before
   pthread_mutex_init(&mu, 0);
   pthread_atfork(mu_lock, mu_unlock, mu_funlock);
   int i, n = 4;