systemctl: add service-log-{level,target} verbs

Heavily inspired by #15622. This adds:
  systemctl service-log-level systemd-resolved
  systemctl service-log-level systemd-resolved info
  systemctl service-log-target systemd-resolved
  systemctl service-log-target systemd-resolved console

We already have systemctl verbs log-level, log-target, and service-watchdogs.
Those two new verbs tie nicely into this scheme.