OS X: kbfs fails when /keybase is not owned by my renumbered uid

Recently I renumbered uids. This usually involves `chown -R username ~username`. I discovered I had to change the ownership of `/keybase` for the kbfs mount to succeed. Strange error messages spammed me until I did this.

I believe this is the single-user case of #2322.

In this case, my problem would likely be solved if `/keybase` were a symlink to `~/Library/Application Support/Keybase/fs` or somewhere else kbfs is mounted in `$HOME`--that’s the only place users can really count on being writable.

Note that placing the real root somewhere private in `$HOME` (or `$XDG_RUNTIME_DIR` aka `/run/user/{uid}`) simplifies the problem of “I don’t want `/keybase`, put it in `~/keybase` please.” #2306

Start by investigating the kbfs mount setup on macOS, including how /keybase ownership and the proposed location under the user's home directory affect a renumbered uid. Review related issues #2322 and #2306. Done means the mount succeeds without manually changing /keybase ownership and the intended location behavior is documented or implemented.

Written by the indexing model from the issue text.