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,
 };

windows/plink.c

@@ -415,7 +415,7 @@ int main(int argc, char **argv)
     if (vt->flags & BACKEND_NEEDS_TERMINAL) {
         fprintf(stderr,
                 "Plink doesn't support %s, which needs terminal emulation\n",
-                vt->displayname);
+                vt->displayname_lc);
         return 1;
     }
 
@@ -441,7 +441,7 @@ int main(int argc, char **argv)
     if (just_test_share_exists) {
         if (!vt->test_for_upstream) {
             fprintf(stderr, "Connection sharing not supported for this "
-                    "connection type (%s)'\n", vt->displayname);
+                    "connection type (%s)'\n", vt->displayname_lc);
             return 1;
         }
         if (vt->test_for_upstream(conf_get_str(conf, CONF_host),

windows/serial.c

@@ -452,7 +452,8 @@ const BackendVtable serial_backend = {
     .unthrottle = serial_unthrottle,
     .cfg_info = serial_cfg_info,
     .id = "serial",
-    .displayname = "Serial",
+    .displayname_tc = "Serial",
+    .displayname_lc = "serial",
     .protocol = PROT_SERIAL,
     .serial_parity_mask = ((1 << SER_PAR_NONE) |
                            (1 << SER_PAR_ODD) |