user/keychain: new package

Keychain is a set of shell scripts (edited by a Perl script) that automates the handling of OpenSSH, OpenPGP, and other security keys.

Given that it's a shell script, I did not include dependencies on OpenSSH or GnuPG since it can be used with one, the other, both, or one of the more exotic options. Its behavior is completely dependent on the user telling it what to use.

See merge request !109