net-p2p/c-lightning: Bump to 0.7.2.1

This bumps net-p2p/c-lightning to version 0.7.2.1.

The ebuild now creates a lightning user and group, installs an OpenRC service that uses /var/lib/lightning as the data directory, and installs a thoroughly commented example config file in /etc/lightning/config. Adding a regular user to the lightning group and symlinking ~/.lightning to /var/lib/lightning allows that user to invoke lightning-cli to control the OpenRC-managed daemon.