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/x/xslurp.c

@@ -39,7 +39,7 @@ void *xslurp(const char *path, size_t *opt_out_size) {
   res = NULL;
   if ((fd = open(path, O_RDONLY)) != -1) {
     if ((size = lseek(fd, 0, SEEK_END)) != -1 &&
-        (res = memalign(PAGESIZE, size + 1))) {
+        (res = memalign(4096, size + 1))) {
       if (size > 2 * 1024 * 1024) {
         fadvise(fd, 0, size, MADV_SEQUENTIAL);
       }