core: when looking for a UID to use for a dynamic UID start with the current owner...

core: when looking for a UID to use for a dynamic UID start with the current owner of the StateDirectory= and friends

Let's optimize dynamic UID allocation a bit: if a StateDirectory= (or
suchlike) is configured, we start our allocation loop from that UID and
use it if it currently isn't used otherwise. This is beneficial as it
saves us from having to expensively recursively chown() these
directories in the typical case (which StateDirectory= does when it
notices that the owner of the directory doesn't match the UID picked).

With this in place we now have the a three-phase logic for allocating a
dynamic UID:

a) first, we try to use the owning UID of StateDirectory=,
   CacheDirectory=, LogDirectory= if that exists and is currently
   otherwise unused.

b) if that didn't work out, we hash the UID from the service name

c) if that didn't yield an unused UID either, randomly pick new ones
   until we find a free one.