iface-modem-location: validate SUPL server address

Devices will expect SUPL server given as either IP:PORT or FQDN:PORT,
so just avoid saying we require a 'URL' because it's not true.

We will use a new helper method to parse and validate user-provided
SUPL server address.

cli/mmcli-modem-location.c

@@ -145,7 +145,7 @@ static GOptionEntry entries[] = {
     },
     { "location-set-supl-server", 0, 0, G_OPTION_ARG_STRING, &set_supl_server_str,
       "Set SUPL server address",
-      "[IP:PORT] or [URL]"
+      "[IP:PORT] or [FQDN:PORT]"
     },
     { "location-inject-assistance-data", 0, 0, G_OPTION_ARG_FILENAME, &inject_assistance_data_str,
       "Inject assistance data in the GNSS module",