core: allow PIDs to be watched by two units at the same time

core: allow PIDs to be watched by two units at the same time

In some cases it is interesting to map a PID to two units at the same
time. For example, when a user logs in via a getty, which is reexeced to
/sbin/login that binary will be explicitly referenced as main pid of the
getty service, as well as implicitly referenced as part of the session
scope.