install: consider globally enabled units as "enabled" for the user

We would not consider symlinks in /etc/systemd/user/*.{wants,requires}/
towards the user unit being "enabled", because the symlinks were not
located in "config" paths. But this is confusing to users, since those units
are clearly enabled and will be started. So let's muddle the definition of
enablement a bit to include the paths only accessible to root when looking for
enabled user units.

Fixes #4432.