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/calls/ktmppath.c

@@ -48,7 +48,7 @@ __attribute__((__constructor__)) static void kTmpPathInit(void) {
   char16_t path16[PATH_MAX];
 
   if ((s = getenv("TMPDIR")) && (n = strlen(s)) < PATH_MAX / 2) {
-    memcpy(kTmpPath, s, n);
+    if (n) memcpy(kTmpPath, s, n);
     if (n && kTmpPath[n - 1] != '/') {
       kTmpPath[n + 0] = '/';
       kTmpPath[n + 1] = 0;