utf8.c: Generalize static fcn return for indeterminate result

utf8.c: Generalize static fcn return for indeterminate result

Prior to this commit, isFF_OVERLONG() returned a boolean, with 0 also
indicating that there wasn't enough information to make a determination.
I realized that I was forgetting that 0 wasn't necessarily definitive
while coding.  By changing the API to return 3 values, forgetting that
won't likely happen.

This and the next several commits change several other functions that
have the same predicament.