Lowercase version of BackendVtable's displayname.

The current 'displayname' field is designed for presenting in the
config UI, so it starts with a capital letter even when it's not a
proper noun. If I want to name the backend in the middle of a
sentence, I'll need a version that starts with lowercase where
appropriate.

The old field is renamed displayname_tc, to avoid ambiguity.

windows/conpty.c

@@ -385,6 +385,7 @@ const BackendVtable conpty_backend = {
     .unthrottle = conpty_unthrottle,
     .cfg_info = conpty_cfg_info,
     .id = "conpty",
-    .displayname = "ConPTY",
+    .displayname_tc = "ConPTY",
+    .displayname_lc = "ConPTY", /* proper name, so capitalise it anyway */
     .protocol = -1,
 };