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.

unix/uxplink.c

@@ -891,8 +891,8 @@ int main(int argc, char **argv)
 
     if (just_test_share_exists) {
         if (!backvt->test_for_upstream) {
-            fprintf(stderr, "Connection sharing not supported for connection "
-                    "type '%s'\n", backvt->name);
+            fprintf(stderr, "Connection sharing not supported for this "
+                    "connection type (%s)'\n", backvt->displayname);
             return 1;
         }
         if (backvt->test_for_upstream(conf_get_str(conf, CONF_host),

unix/uxpty.c

@@ -1601,7 +1601,7 @@ const struct BackendVtable pty_backend = {
     pty_unthrottle,
     pty_cfg_info,
     NULL /* test_for_upstream */,
-    "pty",
+    "pty", "pty",
     -1,
     0
 };

unix/uxser.c

@@ -576,7 +576,7 @@ const struct BackendVtable serial_backend = {
     serial_unthrottle,
     serial_cfg_info,
     NULL /* test_for_upstream */,
-    "serial",
+    "serial", "Serial",
     PROT_SERIAL,
     0
 };