systemctl: add edit verb

It helps editing units by either creating a drop-in file, like
/etc/systemd/system/my.service.d/override.conf, or by copying the
original unit from /usr/lib/systemd/ to /etc/systemd/ if the --full
option is specified.

It invokes an editor on temporary files related to the unit files and
if the editor exited successfully, then it renames the temporary files
to their original names (e.g. my.service or override.conf) and
daemon-reload is invoked.

If the temporary file is empty the modification is canceled.

See https://bugzilla.redhat.com/show_bug.cgi?id=906824