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/mem/strndup.c

@@ -32,7 +32,7 @@ char *strndup(const char *s, size_t n) {
   char *s2;
   size_t len = strnlen(s, n);
   if ((s2 = malloc(len + 1))) {
-    memcpy(s2, s, len);
+    if (len) memcpy(s2, s, len);
     s2[len] = '\0';
     return s2;
   }