regexp-opt GNU "sync"?

Every now and the people run into the differing semantics of regexp-opt
between XEmacs and GNU Emacs.  PITA, will be noticed at runtime only. 
It seems that an easy fix would be to reverse the 4th, optional NON-SHY
argument in the XEmacs version, into SHY instead.

If done this way, using the 3 first arguments to regexp-opt would
provide compatible (not quite identical though) results on both XEmacs
and GNU Emacs.

This, of course would have the consequence of possibly breaking modes
that do ugly detection based on running-xemacs and such instead of the
number of arguments.  No worries, can be fixed in the packages tree. 
But for external packages there's not much we could do, they might
break.

So, this is a poll of opinions whether people would see the change as a
good thing or not.  It's a tradeoff between possibly breaking something
not maintained by us ATM out there and everyone continuing to have to
remember the differences and in practice to cause subtle breakage every
now and then.

Yes, I volunteer to do the grunt work in the packages tree.  I have
identified some places where things needs fixing due to the current API
differences, will submit patches to them instead if this proposed change
is vetoed.