[RFC] remove GNU rcs from FreeBSD 12

hi,

For long we are planning to remove GNU rcs from base, after a failed attempt
before FreeBSD 10.0. Let see where we are to be able to remove it from FreeBSD
12.

GNU rcs is a GPLv2 software with newer version being GPLv3 preventing any
updates/fixes.

From previous discussions there were issues that has been raised in previous
attempts:
- ident(1) is still useful given we still have Keywords in our sources. It has
  been replaced by a BSD Licensed version (enhanced to improve compatibility
  with Subversion Keyword) for FreeBSD 11. So that tool will remain in base
  after removal of GNU rcs.
- etc-update uses merge(1) from GNU rcs, this has been changed in head to use
  diff3 instead.
- rc.subr allows to use rcs for the backup file functionality. This
  functionality is off by default as such I plan to make a warning if rcs is not
  installed and recommand to install rcs from base (or if noone claim using the
  feature I will just remove the functionality and only keep the default
  behaviour aka keep one backup copy).
- people uses rcs to handle configuration files in /etc for example. for those
  multiple compatible alternatives are available in ports:
  * rcs57: a copy of the latest version of GNU rcs in base before removal
    (GPLv2)
  * rcs: latest GNU rcs version (GPLv3)

I haven't gone the direction of importing OpenRCS (BSD licensed version from
OpenBSD) as it needs way more work to be 100% compatible with latest version of
GNU rcs.

How to proceed:
- First turn off GNU rcs by default for a couple of month.
- Totally remove GNU rcs if no blockers has been raised.

Best regards,
Bapt