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/libosmogsm.map b/src/gsm/libosmogsm.map
index 5598859..a72db52 100644
--- a/src/gsm/libosmogsm.map
+++ b/src/gsm/libosmogsm.map
@@ -339,6 +339,7 @@
 osmo_auth_load;
 osmo_auth_register;
 osmo_auth_supported;
+osmo_sub_auth_type_names;
 
 osmo_rsl2sitype;
 osmo_sitype2rsl;