regcomp.c: Revise API for static function

regcomp.c: Revise API for static function

This is to allow future changes.   The function now returns success or
failure, and the created regnode (if any) is set via a parameter
pointer.

I removed the 'register' declaration to get this to work, because
such declarations are considered bad form these days, e.g.,
http://stackoverflow.com/questions/314994/whats-a-good-example-of-register-variable-usage-in-c