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

test/libc/calls/chdir_test.c

@@ -30,6 +30,7 @@ TEST(chdir, efault) {
 }
 
 TEST(chdir, enoent) {
+  ASSERT_SYS(ENOENT, -1, chdir(""));
   ASSERT_SYS(ENOENT, -1, chdir("doesnotexist"));
   ASSERT_SYS(ENOENT, -1, chdir("o/doesnotexist"));
 }