Implement Simon's suggestion of moving DEFAULT_PROTOCOL into a per-backend-

link-module const variable `be_default_protocol' which suggests a sensible
default to the front end (which can ignore it). (DEFAULT_PORT is replaced by a
lookup in the backend[] table.)
Still not pretty, but it does mean that the recent fix for `ssh-default'
doesn't break PuTTYtel.

[originally from svn r2613]