Allow Telnet client to be in binary mode persistently

Before this change, we'd always request a return to Telnet NVT (turn off
binary-TX in both directions) after any file transfer. So although a Telnet
session might be negotiated into binary mode shortly after connection
(e.g., using "telnet -8"), it would be reverted back to NVT mode after any
file transfer.

The request to turn off remote binary-TX after executing external programs
didn't actually accomplish anything since we track the Telnet option states
internally and don't send redundant requests (e.g. the change into a mode
we're already in). External programs aren't expected to send Telnet requests
anyway, so I think this was some holdover from early days of stp/sexyz
development.