pid1: do not use mtime==0 as sign of masking (#4388)

It is allowed for unit files to have an mtime==0, so instead of assuming that
any file that had mtime==0 was masked, use the load_state to filter masked
units.

Fixes https://bugzilla.redhat.com/show_bug.cgi?id=1384150.
(cherry picked from commit ba25d39e)