CHOP-962: Preserve unrecognized user settings variables.

Change LLControlVariable::mPersist from bool to tri-state enum. Its 'true'
value used to mean "persist only if changed from default;" third state now
means "persist regardless of value." Add forcePersist() method to set that.
Replace isSaveValueDefault() method -- used only by logic to determine whether
to save the variable -- with shouldSave() method to encapsulate ALL that logic
rather than only part of it. shouldSave() recognizes PERSIST_ALWAYS state.
When loading an unrecognized control variable from a user settings file, use
forcePersist() to ensure that we later save that variable again.
Tweak one of the unit tests to adjust for new semantics.