Add support for multiple cvs root directories. This allow
to use one script for different cvs trees, e.g.

learn:
cvsweb.cgi?cvsroot=learn

freebsd:
cvsweb.cgi?cvsroot=freebsd

openbsd:
cvsweb.cgi/src/bin/cat/cat.c?cvsroot=openbsd

The default cvs root is `freebsd'.