Assist building and "installing" as non-root.

[jasonriedy]

Many(?) people build and install into a separate DESTDIR to audit what happens before installing more widely.

The location of /etc/keyd in this case is questionable. GNU-style installation would place this in $(PREFIX)/etc/keyd. That would require more work on the documentation, and that location is more for distributed configuration files.

Also: Remember systemctl daemon-reload.