shared, systemctl: teach is-enabled to show installation targets

shared, systemctl: teach is-enabled to show installation targets

It may be desired by users to know what targets a particular service is
installed into. Improve user friendliness by teaching the is-enabled
command to show such information when used with --full.

This patch makes use of the newly added UnitFileFlags and adds
UNIT_FILE_DRY_RUN flag into it. Since the API had already been modified,
it's now easy to add the new dry-run feature for other commands as
well. As a next step, --dry-run could be added to systemctl, which in
turn might pave the way for a long requested dry-run feature when
running systemctl start.