[ssh] Add config for macOS keychain
[ssh] Add config for macOS keychain

This is the [recommended config][1] in macOS >= 10.12.2 for loading keys
into ssh-agent and storing passphrases in the keychain.

[1]:https://developer.apple.com/library/content/technotes/tn2449/_index.html

reshleman/dotfiles,reshleman/dotfiles