Add a forward compatability hook so that current cvs can recognize future

Add a forward compatability hook so that current cvs can recognize future
CVSROOT/config file options that control keyword expansion.   cvs-1.12 has
its own $Id$ expansion controls and they're configured in CVSROOT/config
rather than CVSROOT/options.  The problem is that current cvs-1.11.x
doesn't understand the future keywords.....

Add trivial forward support for the new keywords for when cvs-1.12
hits the tree down the road. CVSROOT/options won't be going away - cvsup
uses it.