auth: add value_strings for osmo_sub_auth_type, comment on osmo_auth_alg_name()

Add osmo_sub_auth_type_names[] and osmo_sub_auth_type_name().

Also add a hint to enum osmo_auth_algo's API doc that osmo_auth_alg_name()
already exists (it is defined further below).

Change-Id: I652a929bcd11c694d86812fb03d0a1cbd985efda
diff --git a/src/gsm/auth_core.c b/src/gsm/auth_core.c
index 4156695..738e860 100644
--- a/src/gsm/auth_core.c
+++ b/src/gsm/auth_core.c
@@ -221,4 +221,11 @@
 	return get_string_value(auth_alg_vals, name);
 }
 
+const struct value_string osmo_sub_auth_type_names[] = {
+	{ OSMO_AUTH_TYPE_NONE, "None" },
+	{ OSMO_AUTH_TYPE_GSM, "GSM" },
+	{ OSMO_AUTH_TYPE_UMTS, "UMTS" },
+	{ 0, NULL }
+};
+
 /*! @} */