Revert "Merge remote-tracking branch 'masterzen/tickets/2.7.x/3757' into 2.7.x"

This reverts commit 86a806f595f8b7bb280c8c445eef51dfd71bf39d, reversing
changes made to ac81771a9cc70b5fb58f86439ebb0585ce92d4e8.

See tickets #3757, #12844, #4836, #11057. These changes were deemed to
be breaking public APIs, and thus are being reverted from 2.7.x. They'll
be reintroduced in 3.x.

(The changes in question were relating to how puppet agent creates /
manages its lockfiles.)

Conflicts:

lib/puppet/util/pidlock.rb
spec/unit/agent_spec.rb
spec/unit/util/pidlock_spec.rb