systemctl: resolve symlinks when finding unit paths (#4545)

Otherwise we think the alias is the real unit, and may edit/cat the
wrong unit.

Before this patch:
 $ systemctl edit autovt@ # creates dropin in /etc/systemd/system/autovt@.service.d
 $ systemctl cat autovt@ | grep @.service
 # /lib/systemd/system/autovt@.service
 # that serial gettys are covered by serial-getty@.service, not this
 # /etc/systemd/system/autovt@.service.d/override.conf
 $ systemctl cat getty@ | grep @.service
 # /lib/systemd/system/getty@.service
 # that serial gettys are covered by serial-getty@.service, not this

After this patch
 $ systemctl edit autovt@ # creates dropin in /etc/systemd/system/getty@.service.d
 $ systemctl cat autovt@ | grep @.service
 # /usr/lib/systemd/system/getty@.service
 # that serial gettys are covered by serial-getty@.service, not this
 # /etc/systemd/system/getty@.service.d/override.conf
 systemctl cat getty@ | grep @.service
 # /usr/lib/systemd/system/getty@.service
 # that serial gettys are covered by serial-getty@.service, not this
 # /etc/systemd/system/getty@.service.d/override.conf