(#19373) Add explicit "telnet" protocol for repl

Previously, the nrepl protocol would work over telnet. Recent versions,
however, have changed their defaults so now they default to using the
faster, more compact binary "bencode" protocol. That's great for IDEs
and editors, but not so great for raw socket access via telnet.

I've added a new repl protocol type, "telnet", that reconfigures the
nrepl transport to operate correctly over a TTY.
Update REPL guide to mention new telnet protocol
Merge pull request #494 from grimradical/nrepl-telnet

(#19373) Add explicit "telnet" protocol for repl