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/chdir-nt.c

@@ -31,6 +31,7 @@ textwindows int sys_chdir_nt(const char *path) {
   int e, ms, err, len;
   char16_t path16[PATH_MAX], var[4];
   if ((len = __mkntpath(path, path16)) == -1) return -1;
+  if (!len) return enoent();
   if (len && path16[len - 1] != u'\\') {
     if (len + 2 > PATH_MAX) return enametoolong();
     path16[len + 0] = u'\\';