core: refactor compare_job_priority()

core: refactor compare_job_priority()

Let's move it out of cgroup.[ch]. The function primarily compares the
priority values for units, hence let's move the core of it into a new
function unit_compare_priority() in unit.[ch], and then make
compare_job_priority() a local wrapper for it in manager.[ch]

Shorten the code a bit while we are at it.