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.

unix/pty.c

@@ -1597,6 +1597,7 @@ const BackendVtable pty_backend = {
     .unthrottle = pty_unthrottle,
     .cfg_info = pty_cfg_info,
     .id = "pty",
-    .displayname = "pty",
+    .displayname_tc = "pty",
+    .displayname_lc = "pty",
     .protocol = -1,
 };