core: allow 'SetUnitProperties()' to run on inactive units too

core: allow 'SetUnitProperties()' to run on inactive units too

'set-property' has been primarly designed to change some properties of
*active* units.

However it can easily work on inactive units as well. In that case
changes are only saved in a drop-in for futur uses and changes will be
effective when unit will be started.

Actually it already works on inactive units but that was not
documented and not fully supported. Indeed the inactive units had to
be known by the manager otherwise it was reported as not loaded:

 $ systemctl status my-test.service
 * my-test.service - My Testing Unit
    Loaded: loaded (/etc/systemd/system/my-test.service; static; vendor preset: disabled)
   Drop-In: /etc/systemd/system/my-test.service.d
    Active: inactive (dead)

 $ systemctl set-property my-test.service MemoryLimit=1000000
 Failed to set unit properties on my-test.service: Unit my-test.service is not loaded.

[ Note: that the unit load state reported by the 'status' command
  might be confusing since it claimed the unit as loaded but
  'set-property' reported the contrary. ]

One can possibily workaround this by making the unit a dependency of
another active unit so the manager will keep it around:

 $ systemctl add-wants multi-user.target my-test.service
 Created symlink from /etc/systemd/system/multi-user.target.wants/my-test.service to /etc/systemd/system/my-test.service.

 $ systemctl set-property my-test.service MemoryLimit=1000000

 $ systemctl status my-test.service
 * my-test.service - My Testing Unit
    Loaded: loaded (/etc/systemd/system/my-test.service; enabled; vendor preset: disabled)
   Drop-In: /etc/systemd/system/my-test.service.d
            `-50-MemoryLimit.conf
    Active: inactive (dead)

This patch simply forces 'SetUnitProperties()' to load the unit if
it's not already the case.

It also documents the fact that 'set-property' can be used on inactive
units.