Fix bug in which the SSH-only tools (pscp, psftp) did not honour a

Fix bug in which the SSH-only tools (pscp, psftp) did not honour a
nonstandard port number when loading a saved session.

Occurs because those tools include be_none.c which defines no entries
in backends[] at all, as a result of which settings.c doesn't
recognise the word 'ssh' in the saved session's protocol field and
instead sets the protocol to something idiotic - which _then_ means
that when pscp.c forces the protocol to PROT_SSH, it also resets the
port number as it would when overriding a saved session specifying a
protocol other than SSH.

The immediate solution is to define a new be_ssh.c citing only
ssh_backend, and include that in the SSH-only tools. However, I wonder
if a better approach (perhaps when I redesign session loading and
saving) would be not to be so clever, and just have all the tools
contain a complete list of known protocol names for purposes of
understanding what's in the saved session data, and complain if you
try to use one they don't know how to actually speak.

[originally from svn r9254]