Revoke privilege on *.* to role@''; also works

Also added syntax for GRANT privilege TO { role } command

TODO: Use an index search on the table, instead of scanning through it.

This change only updates _in memory_ structures. 

Added extra comments to explain the ACL_USER_BASE flags usage, as well as fix an issue with get_role_access.

The bug caused roles rights to not be propagated if a push on the stack happened. The newly finished neighbour
was never reevaluated.

ACL_ROLE now inherits ACL_USER_BASE

Fixed possible failing condition due to different order of table rows

Moved fields corresponding to role entries to the ACL_ROLE class.

from the grant role to role graph.

resets the initial role access bits.

Also called show grants before and after set role. Unfortunately the role
privileges printing are not implemented yet.

The testcase checks to see if the privileges are set accordingly to a newly
created user. This is the most general usecase.

The testcase checks to see if the create user command sets the is_role column
to 'N' by default