core: skip deps on oomd if v2 or memory unavailable

core: skip deps on oomd if v2 or memory unavailable

Fixes: https://bugzilla.redhat.com/show_bug.cgi?id=2055664
Fixes: https://bugzilla.redhat.com/show_bug.cgi?id=2172146
User report that systemd repeatedly logs about not being able to start oomd
when booted with v1:

Feb 20 16:52:33   systemd[1]: systemd-oomd.service - Userspace Out-Of-Memory (OOM) Killer was skipped because of a failed condition check (ConditionControlGroupController=v2).
Feb 20 16:52:34   systemd[1]: systemd-oomd.service - Userspace Out-Of-Memory (OOM) Killer was skipped because of a failed condition check (ConditionControlGroupController=v2).
Feb 20 16:52:34   systemd[1]: systemd-oomd.service - Userspace Out-Of-Memory (OOM) Killer was skipped because of a failed condition check (ConditionControlGroupController=v2).
Feb 20 16:52:34   systemd[1]: systemd-oomd.service - Userspace Out-Of-Memory (OOM) Killer was skipped because of a failed condition check (ConditionControlGroupController=v2).
Feb 20 16:52:34   systemd[1]: systemd-oomd.service - Userspace Out-Of-Memory (OOM) Killer was skipped because of a failed condition check (ConditionControlGroupController=v2).
Feb 20 16:52:34   systemd[1]: systemd-oomd.service - Userspace Out-Of-Memory (OOM) Killer was skipped because of a failed condition check (ConditionControlGroupController=v2).
Feb 20 16:52:34   systemd[2067491]: Queued start job for default target default.target.
Feb 20 16:52:34   systemd[1]: systemd-oomd.service - Userspace Out-Of-Memory (OOM) Killer was skipped because of a failed condition check (ConditionControlGroupController=v2).
Feb 20 16:52:34   systemd[2067491]: Created slice app.slice - User Application Slice.
Feb 20 16:52:34   systemd[1]: systemd-oomd.service - Userspace Out-Of-Memory (OOM) Killer was skipped because of a failed condition check (ConditionControlGroupController=v2).

systemd-oomd.service that pulls systemd-oomd.socket in (because it requires
it); systemd-oomd.service itself is pulled by user@.service because
systemd-oomd package installs an override config file that sets
ManagedOOMMemoryPressure=kill.

Add a check to the code that adds the implicit dependency to skip the
dep if we cannot start it. The check is done exactly the same as in oomd
itself.