pid1: order jobs that execute processes with lower priority

pid1: order jobs that execute processes with lower priority

We can meaningfully compare jobs for units which have cpu weight or nice set.
But non-exec units those have those set.

Starting non-exec jobs first allows us to get them out of the queue quickly,
and consider more jobs for starting.

If we have service A, and socket B, and service C which is after socket B,
and we want to start both A and C, and C has higher cpu weight, if we get
B out of the way first, we'll know that we can start both A and C, and we'll
start C first.

Also invert the comparisons using CMP() so they are always done left vs. right,
and negate when returning instead.

Follow-up for da8e178296.