Permit protocol selection in file transfer tools.

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.

be_ssh.c

@@ -1,8 +1,7 @@
 /*
- * Linking module for programs that are restricted to only using SSH
- * (pscp and psftp). These do not support selection of backend, but
- * must still have a backends[] array mentioning SSH because
- * settings.c will want to consult it during session load.
+ * Linking module for programs that are restricted to only using
+ * SSH-type protocols (pscp and psftp). These still have a choice of
+ * two actual backends, because they can also speak PROT_SSHCONN.
  */
 
 #include <stdio.h>
@@ -12,5 +11,6 @@ const int be_default_protocol = PROT_SSH;
 
 const struct BackendVtable *const backends[] = {
     &ssh_backend,
+    &sshconn_backend,
     NULL
 };