core: move pid watch/unwatch logic of the service manager to pidfd

core: move pid watch/unwatch logic of the service manager to pidfd

This makes sure unit_watch_pid() and unit_unwatch_pid() will track
processes by pidfd if supported. Also ports over some related code.
Should not really change behaviour.

Note that this does *not* add support waiting for POLLIN on the pidfds
as additional exit notification. This is left for a later commit (this
commit is already large enough), in particular as that would add new
logic and not just convert existing logic.