(#12844) Agent lockfiles: backwards compatibility with 2.7.10/2.7.11

In 2.7.10 there was a change in behavior introduced with regards
to agent lockfiles. Basically we split the concept of "an agent
is currently running" apart from the concept of "the agent has
been administratively disabled" by using 2 different lockfiles.

That change was determined to have broken compatibility with
mcollective, so it has been reverted as of 2.7.12.

This commit provides backwards compatibility between 2.7.12+
and 2.7.10/2.7.11 for cases where a user may have administratively
disabled their agent and then upgraded to a newer version of puppet.