dropin: when looking for dropins for a unit, also look within "-" prefix unit dirs

This extends the logic by which we look for drop-ins for unit files when
loading them. Previously for a unit "foo-quux-bar.service" we'd look in
a directory "foo-quux-bar.service.d" accompanying it for extension
dropins. With this change we'll additionally look in:
"foo-quux-.service.d" and "foo-.service.d", i.e. we'll truncate the unit
name after every dash.

This is an alternative to templating for many services, as it permits
configuring defaults for sets of units that all use the same prefix in
the unit name. This is particularly useful in slice, mount and
automount units which reflect a hierarchy of concepts, as it permits
setting defaults for specific subsets of the tree. For example, in order
to provide every user with a memory of 1G it's now possible to do:

    # mkdir -p /etc/systemd/system/user-.slice.d
    # cat > /etc/systemd/system/user-.slice.d/50-memory.conf << EOF
    [Slice]
    MemoryMax=1G
    EOF
    # systemctl daemon-reload

This makes use of the fact that every user gets his own slice unit when
logging in, named "user-$UID.slice".

This doesn't precisely provide what is requested in #2556, but it does
provide equivalent functionality.

Fixes: #2556
See: #3504 #7599