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: 173728
Phabric: D995
Reviewed by: markj, markm
MFC after: 2 weeks
Sponsored by: EMC / Isilon Storage Division
Revert WiP to contrib/tzcode accidentally committed with r274364