Add baud rate support to telnet(1)

This implements part of RFC-2217

It's based off a patch originally written by Sujal Patel at Isilon, and
contributions from other Isilon employees.

PR: [[ https://bugs.freebsd.org/173728 | 173728 ]]
Phabric: D995
Reviewed by: markj, markm
Sponsored by: EMC / Isilon Storage Division