Revise subfigure counter ownership

Currently the subcaption package claims ownership of the "sub" counters via \DeclareCaptionSubType. This should be changed so mixed operation with subfig or memoir is possible:

• Only claim ownership via \DeclareCaptionSubType is the sub-counter is not defined yet (otherwise: \subcaption@Info)
• If \setcaptionsubtype is used but we do not own the sub-counter: \subcaption@Error(This error message should contain the owner of the sub-counter, i.e. subfig package or memoir (if this could be detected) or unknown source)
• The tests if either subfigure or subfig is loaded should be removed