devel/cvs: enslave devel/cvs+ipv6 to avoid bitrot

• devel/cvs: allow slave ports (?= vars)
• devel/cvs: define CONFLICTS against the slave and -devel
• devel/cvs: adjust for easier patching source (i.e. no ../cvs/ prefix)
• devel/cvs+ipv6: move most logic as IPV6 option to devel/cvs
• devel/cvs+ipv6: don't define HAVE_GETGROUPS twice or unused ENABLE_IPV6
• devel/cvs+ipv6: drop devel/cvsnt from CONFLICTS after 4.6 years being gone
• devel/cvs+ipv6: bump PORTREVISION for changes inherited from devel/cvs
• Convert CONFLICTS into CONFLICTS_INSTALL

PR: 199542
Reviewed by: bapt
Approved by: maintainer timeout (5 months)
Differential Revision: https://reviews.freebsd.org/D3636