This replaces the pure radio-button setup that we've always had on the
Session config panel.

Since the last release, that set of radio buttons has been getting out
of hand. We've added two new protocols (SUPDUP, and the 'bare
ssh-connection' aka psusan protocol), neither of which is mainstream
enough to be a sensible thing to wave at all users on the front page
of the config GUI, so that they perhaps start wondering if that's the
protocol they want to use, or get sidetracked by going and looking it
up.

The replacement UI still has radio buttons, but only for the most
common protocols, which will typically be SSH and serial. Everything
else is relegated to a drop-down list sitting next to a third radio
button labelled "Other".

In every be_* module providing a backends[] list, there's also a
variable n_ui_backends which indicates how many of the backends ought
to appear as first-level radio buttons.

(Credit where due: this patch is a joint effort between Jacob and me,
and is one of those rare cases where it would be nice to be able to
put both our names into the Author field of the commit. Failing that,
I can at least mention it here.)

PSCP and PSFTP can only work over a protocol enough like SSH to be
able to run subsystems (or at the very least a remote command, for
old-style PSCP). Historically we've implemented this restriction by
having them not support any protocol-selection command-line options at
all, and hardwiring them to instantiating ssh_backend.

This commit regularises them to be more like the rest of the tools.
You can select a protocol using the appropriate command-line option,
provided it's a protocol in those tools' backends[] array. And the
setup code will find the BackendVtable to instantiate by the usual
method of calling backend_vt_from_proto.

Currently, this makes essentially no difference: those tools link in
be_ssh.c, which means the only supported backend is SSH. So the effect
is that now -ssh is an accepted option with no effect, instead of
being rejected. But it opens the way to add other protocols that are
SSH-like enough to run file transfer over.

Now they're all called FooVtable, instead of a mixture of that and
Foo_vtable.

Nearly every part of the code that ever handles a full backend
structure has historically done it using a pair of pointer variables,
one pointing at a constant struct full of function pointers, and the
other pointing to a 'void *' state object that's passed to each of
those.

While I'm modernising the rest of the code, this seems like a good
time to turn that into the same more or less type-safe and less
cumbersome system as I'm using for other parts of the code, such as
Socket, Plug, BinaryPacketProtocol and so forth: the Backend structure
contains a vtable pointer, and a system of macro wrappers handles
dispatching through that vtable.

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]