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

libc/intrin/setenv.c

@@ -30,6 +30,7 @@
  * @raise EINVAL if `name` is empty or contains `'='`
  * @raise ENOMEM if we require more vespene gas
  * @see putenv(), getenv()
+ * @threadunsafe
  */
 int setenv(const char *name, const char *value, int overwrite) {
   int rc;