ref/services/mounttable: Prevent users from locking themselves out
of the mounttable.

This closes veyron/release-issues#1862

If the caller's tries to Set an acl that removes them from Admin, the caller's
original Admin blessings will be retained.

Change-Id: Ib7c82b4e27ce7e4f0d99f932f734e194c66bc01c
2 files changed