regcomp.c: Fix typo in comment
regcomp.c: Tighten up synthetic start class for Unicode characters

When creating a synthetic start class (SSC) for EXACTFish nodes, in many
instances prior to this patch we punted and assumed any code point could
be matched.  This patch actually goes to the trouble of figuring out
what Unicode code points could possibly match, and uses those.  The only
time it punts now is when the node starts with a string of multiple
characters that together could be the fold of a single one.  It's too
complicated getting all the possibilities for these, as there could be
overlapping sequences that work.