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/runtime/clone.c

@@ -644,7 +644,7 @@ errno_t clone(void *func, void *stk, size_t stksz, int flags, void *arg,
   if (!func) {
     rc = EINVAL;
   } else if (!IsTiny() &&
-             ((flags & CLONE_VM) && (stksz < PAGESIZE || (stksz & 15)))) {
+             ((flags & CLONE_VM) && (stksz < 4096 || (stksz & 15)))) {
     rc = EINVAL;
   } else if (IsAsan() &&
              (((flags & CLONE_SETTLS) && !__asan_is_valid(tls, 64)) ||