FTS Support

[kateinoigakukun]

wasi-libc does not include fts(3) implementation now, and it prevents us from reusing other platforms' code while porting swift-foundation to WASI.

Can we ship https://github.com/void-linux/musl-fts licensed under BSD 3-Clause as a part of wasi-libc?

The implementation is well-matured and well-used in the Alpine ecosystem. The only change we need to apply to the implementation is not to depend on fchdir by making FTS_NOCHDIR default.