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/putenv.c

@@ -93,6 +93,7 @@ int PutEnvImpl(char *s, bool overwrite) {
  * @return 0 on success, or non-zero w/ errno on error
  * @raise ENOMEM if we require more vespene gas
  * @see setenv(), getenv()
+ * @threadunsafe
  */
 int putenv(char *s) {
   int rc;