Do not report masked units as changed (#2921)

Do not report masked units as changed (#2921)

* core/unit: extract checking of stat paths into helper function

The same code was repeated three times.

* core: treat masked files as "unchanged"

systemctl prints the "unit file changed on disk" warning
for a masked unit. I think it's better to print nothing in that
case.

When a masked unit is loaded, set mtime as 0. When checking
if a unit with mtime of 0 needs reload, check that the mask
is still in place.

* test-dnssec: fix build without gcrypt

Also reorder the test functions to follow the way they are called
from main().