Deprecate telnet daemon

The telnetd codebase is old, unmaintained, and has a number of quality
issues. Users wishing to provide telnetd service should find a
maintained implementation. The telnet client is NOT deprecated as it
is lower risk.

Reviewed by: pauamma, kevans, kp, melifaro
MFC After: 3 days
Differential Revision: https://reviews.freebsd.org/D36619