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/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
 };