cgroup: Reduce unit_get_ancestor_disable_mask use

cgroup: Reduce unit_get_ancestor_disable_mask use

The usage in unit_get_own_mask is redundant, we only need apply
disable_mask at the end befor application, i.e. calculating enable or
target mask.

(IOW, we allow all configurations, but disabling affects effective
controls.)

Modify tests accordingly and add testing of enable mask.

This is intended as cleanup, with no effect but changing unit_dump
output.