Ungarrison entities using cmpGarrisonable.
Follow-up to r24977 / rP24977.
This moves responsibilities even further to the garrisoning entity.
Fixes the garrison flag when renaming entities cannot garrison (#5906).
Allows easy fixing of #6087.
Ticket: #6081
Differential revision: D3628
Comments by: @Stan, @wraitii (also in length on IRC)