Fix bugs and make improvements

- Get clone() working on FreeBSD
- Increase some Python build quotas
- Add more atomic builtins to chibicc
- Fix ASAN poisoning of alloca() memory
- Make MODE= mandatory link path tinier
- Improve the examples folder a little bit
- Start working on some more resource limits
- Make the linenoise auto-complete UI as good as GNU readline
- Update compile.com, avoiding AVX codegen on non-AVX systems
- Make sure empty path to syscalls like opendir raises ENOENT
- Correctly polyfill ENOENT vs. ENOTDIR on the New Technology
- Port bestline's paredit features to //third_party/linenoise
- Remove workarounds for RHEL 5.0 bugs that were fixed in 5.1

libc/calls/mkntpathat.c

@@ -29,6 +29,7 @@ int __mkntpathat(int dirfd, const char *path, int flags,
   char16_t dir[PATH_MAX];
   uint32_t dirlen, filelen;
   if ((filelen = __mkntpath2(path, file, flags)) == -1) return -1;
+  if (!filelen) return enoent();
   if (file[0] != u'\\' && dirfd != AT_FDCWD) { /* ProTip: \\?\C:\foo */
     if (!__isfdkind(dirfd, kFdFile)) return ebadf();
     dirlen = GetFinalPathNameByHandle(g_fds.p[dirfd].handle, dir, ARRAYLEN(dir),