core: introduce a "control" unit file directory

This patch adds a concept of a "control" unit file directory, which is supposed
to be used as place for unit file drop-ins created by "systemctl set-property"
(note that this directory is not actually hooked up to "systemctl set-property"
yet, that's coming in a later patch).

The rationale for this: previously changes made by the user and by "systemctl
set-property" were done in the same directory, which made semantics very
unclear: the changes made by "systemctl set-property" were applied instantly,
and their drop-ins only written to not lose settings on a later "systemctl
daemon-reload", while drop-ins made by the user would only be in effect after
"systemctl daemon-reload". This is particular problematic as the changes made
by "systemctl set-property" would really apply immediately without any respect
for the unit search path. This meant that using "set-property" could have an
effect that is lsot as soon as "daemon-reload" is issued, in case there was a
"later" drop-in already in place.

With this change the directories are seperated, and the "control" directory
always takes the highest priority of all, to avoid any confusion.