Add support for running not only as PID 1

[troglobit]

The process monitoring capabilities of Finit could be beneficial in use-cases other than just PID 1. One such example use-case is http://supervisord.org/

• Configuration in ~/.config/finit.conf, requiring use of finit_rcsd path
• Optionally configuration via -f path/to/.conf
• API socket in /run/user/$(id -u)/finit/socket
• Resource file in /run/user/$(id -u)/finit/.initrc

Note, this requires something similar to pam_systemd(8) to notify Finit of users logging in.