interpret domain '*' as 'allow any domain'

Make the 'domain NAME' vty doc more descriptive, and add the hint that '*'
means any domain.

In check_domain_name(), exit early in success if the configured domain name is
'*'.

(Do not implement other wildcard functionality for partial matches or the
like, just the single '*'.)

Related: OS#3490
Change-Id: Ie0220c88d2f5cee15f2a90390b3c4590ac61d5eb
diff --git a/src/libosmo-mgcp/mgcp_vty.c b/src/libosmo-mgcp/mgcp_vty.c
index ad462b7..00af3bd 100644
--- a/src/libosmo-mgcp/mgcp_vty.c
+++ b/src/libosmo-mgcp/mgcp_vty.c
@@ -1208,7 +1208,9 @@
 
 DEFUN(cfg_mgcp_domain,
       cfg_mgcp_domain_cmd,
-      "domain NAME", "domain\n" "qualified domain name\n")
+      "domain NAME",
+      "Set the domain part expected in MGCP messages' endpoint names\n"
+      "Qualified domain name expected in MGCP endpoint names, or '*' to accept any domain\n")
 {
 	osmo_strlcpy(g_cfg->domain, argv[0], sizeof(g_cfg->domain));
 	return CMD_SUCCESS;