regcomp.c: Make invlist_search() usable from re_comp.c

regcomp.c: Make invlist_search() usable from re_comp.c

This was a static function which I couldn't get to be callable from the
debugging version of regcomp.c.  This makes it public, but known only
in the regcomp.c source file.  It changes the name to begin with an
underscore so that if someone cheats by adding preprocessor #defines,
they still have to call it with the name that convention indicates is a
private function.