devel/cvs: enslave devel/cvs+ipv6

[jbeich]

Summary

In order to take advantage of rP361895, rP361899, rP384063 with future in mind. @sumikawa (maintainer) timed out for 5 months but actually inactive for more. Let's check sanity with my mentor.

• devel/cvs: allow slave ports (?= vars)
• devel/cvs: define CONFLICTS against the slave and -devel
• devel/cvs: adjust for easier patching source (i.e. avoid ../cvs)
• 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: bump PORTREVISION changes inherited from devel/cvs
• Drop devel/cvsnt from CONFLICTS after 4.6 years being gone
• Convert CONFLICTS into CONFLICTS_INSTALL

new since v1 (on bugzilla):

• Rebase after rP388950, rP396685
• Convert CONFLICTS into CONFLICTS_INSTALL

Test Plan

poudriere logs in ports/199542.

$ (cd devel/cvs; portlint -C)
WARN: Makefile: unless this is a master port, MAINTAINER has to be set by "=", not by "?=".
WARN: Makefile: unless this is a master port, COMMENT has to be set by "=", not by "?=".
WARN: files/patch-cvs.info.texinfo-5.x: patch was not generated using ``make makepatch''.  It is recommended to use ``make makepatch'' when you need to [re-]generate a patch to ensure proper patch format.
WARN: files/patch-cvs__Makefile: patch was not generated using ``make makepatch''.  It is recommended to use ``make makepatch'' when you need to [re-]generate a patch to ensure proper patch format.
0 fatal errors and 4 warnings found.

$ (cd devel/cvs+ipv6; portlint -C)
WARN: Makefile: KERBEROS is listed in OPTIONS_DEFINE, but no PORT_OPTIONS:MKERBEROS appears.
FATAL: Makefile: extra item "CONFLICTS_INSTALL" placed in the LICENSE section.
WARN: ../cvs/files/patch-cvs.info.texinfo-5.x: patch was not generated using ``make makepatch''.  It is recommended to use ``make makepatch'' when you need to [re-]generate a patch to ensure proper patch format.
WARN: ../cvs/files/patch-cvs__Makefile: patch was not generated using ``make makepatch''.  It is recommended to use ``make makepatch'' when you need to [re-]generate a patch to ensure proper patch format.
1 fatal error and 3 warnings found.

[bapt]

This is very good improvements!