CHOP-962: Make LLControlVariable::setPersist() accept only enum.

Initial change made LLControlVariable::mPersist an enum, but retained
bool/BOOL public API. setPersist(true) set one value, setPersist(false) set
another, forcePersist() set the third. Per code review, expose enum to public,
make setPersist() (and LLControlVariable constructor, and LLControlGroup::
declareControl(), and all the LLControlGroup::declareMumble() methods, and all
the unit-test dummy declareMumble() method bodies) accept that enum. Remove
forcePersist(). Fix calls to LLControlGroup::declareMumble() accordingly.
Also rename PERSIST_YES to PERSIST_NONDFT, also per code review.