oomd: loosen the restriction on ManagedOOMPreference

oomd: loosen the restriction on ManagedOOMPreference

The ManagedOOMPreference property is only honored on cgroups which are
owned by root. This precludes anyone from setting ManagedOOMPreference
on cgroups managed by user managers.

Loosen this restriction in the following way: when processing a
monitored cgroup for kill candidates, honor the ManagedOOMPreference
setting if the monitored cgroup and cgroup candidate are owned by the
same user. This allows unprivileged users to configure
ManagedOOMPreference on their cgroups without affecting the kill
priority of ancestor cgroups.

N.B. that since swap kill operates globally to kill the largest
candidate, it is not appropriate to apply this logic to the swap kill
scenario. Therefore, the existing restriction on ManagedOOMPreference
will remain when calculating candidates for swap kill.

Add a new function, oomd_fetch_cgroup_oom_preference, to assist with
this new logic. To simplify things, move the `user.oomd_{avoid,omit}`
xattr reads to this function so that the xattr reads and uid checks are
performed all at once.