Give BackendVtable separate id and displayname fields.

The previous 'name' field was awkwardly serving both purposes: it was
a machine-readable identifier for the backend used in the saved
session format, and it was also used in error messages when Plink
wanted to complain that it didn't support a particular backend. Now
there are two separate name fields for those purposes.

ssh.c

@@ -1190,7 +1190,7 @@ const struct BackendVtable ssh_backend = {
     ssh_unthrottle,
     ssh_cfg_info,
     ssh_test_for_upstream,
-    "ssh",
+    "ssh", "SSH",
     PROT_SSH,
     22
 };