core: log about /var/run/ prefix used in PIDFile=, patch it to be /run instead

core: log about /var/run/ prefix used in PIDFile=, patch it to be /run instead

In a way this is a follow-up for
a2d1fb882c4308bc10362d971f333c5031d60069, but adds a similar warning for
PIDFile=.

There's a much stronger case for doing this kind of notification in
tmpfiles.d (since it helps relating lines to each other for the purpose
of merging them). Doing this for PIDFile= is mostly about being
systematic and copying tmpfiles.d/ behaviour here.

While we are at it, let's also support relative filenames in PIDFile=
now, and prefix them with /run, to make them absolute.

Fixes: #10657