metadata: add new item type KDBUS_ITEM_AUXGROUPS (ABI break)

Add new data type to dump the auxiliary groups the sending task is a member
of.
test: add test code for KDBUS_ITEM_AUXGROUPS
metadata: return -EFAULT if get_task_mm(current) fails
Merge remote-tracking branch 'kdbus/master' into 3.15/kdbus