deprecate RIP

Per https://wiki.freebsd.org/DeprecationPlan, deprecate RIP in 15.0R,
with the intention of removing it in 16.0R.

Add a note to the manpages of routed(8), rdisc(8), rtquery(8),
route6d(8) and rip6query(8) noting that they will be removed
in a future release.

RIP has been obsolete for over 20 years and no one is running it on
modern networks. If someone really needs RIP, there are alternatives
such as 'bird' or 'quagga' available from ports; there is no need to
ship it in base.

Relnotes: yes
Reviewed by: des
Approved by: des (mentor)
Differential Revision: https://reviews.freebsd.org/D50292

(cherry picked from commit 4b7cdb5a420f518fbe1968a6916b6d017f54025d)