job: change result field for log message about job result RESULT= → JOB_RESULT=

job: change result field for log message about job result RESULT= → JOB_RESULT=

So, currently, some of the structured log messages we generated based on
jobs carry the result in RESULT=, and others in JOB_RESULT=. Let's
streamline this, as stick to JOB_RESULT= in one place.

This is kind of an API break, but given that currently most software has
to check both fields anyway, I think we can get away with it.

Why unify on JOB_RESULT= rather than RESULT=? Well, we manage different
types of result codes in systemd. Most importanlty besides job results
there are also service results, and we should be explicit in what we
mean here.