core: make unit_has_mask_realized() consider controller enable state

core: make unit_has_mask_realized() consider controller enable state

unit_has_mask_realized() determines whether the specified unit has its cgroups
set up properly given the desired target_mask; however, on the unified
hierarchy, controllers need to be enabled explicitly for children and the mask
of enabled controllers can deviate from target_mask.  Only considering
target_mask in unit_has_mask_realized() can lead to false positives and
skipping enabling the requested controllers.

This patch adds unit->cgroup_enabled_mask to track which controllers are
enabled and updates unit_has_mask_realized() to also consider enable_mask.

Signed-off-by: Tejun Heo <htejun@fb.com>