ctld: Move the pidfile handle out to a global variable

This ensures it will be destroyed (removing the associated pidfile)
anytime the process exits, including from exit(3) calls. This fixes
a few places that would "leak" the pidfile on certain errors.

This also removes the need for some convoluted logic where
configuration objects would hand-off ownership of the pidfile handle
from the old configuration to the new configuration.

Reviewed by: asomers
Sponsored by: Chelsio Communications
Differential Revision: https://reviews.freebsd.org/D56527

(cherry picked from commit aad4fec5d7e260a789456a8cc82b293064fab5f3)