Advertise in-band registration support.

Implement optional registration support advertisement according to
XEP-0077 sec8:

<features>
    <register xmlns='http://jabber.org/features/iq-register'/>
</features>
---
 plugins/mod_register.lua |   12 ++++++++++++
 1 files changed, 12 insertions(+), 0 deletions(-)

1 files changed, 12 insertions, 0 deletions

diff --git a/plugins/mod_register.lua b/plugins/mod_register.lua
index 25c09dfa..b4683b58 100644
--- a/plugins/mod_register.lua
+++ b/plugins/mod_register.lua
@@ -22,6 +22,18 @@ local compat = module:get_option_boolean("registration_compat", true);
 
 module:add_feature("jabber:iq:register");
 
+local register_stream_feature = st.stanza("register", {xmlns="http://jabber.org/features/iq-register"}):up();
+module:hook("stream-features", function(event)
+        local session, features = event.origin, event.features;
+
+	-- Advertise registration to unauthorized clients only.
+	if module:get_option("allow_registration") == false or session.type ~= "c2s_unauthed" then
+		return
+	end
+
+	features:add_child(register_stream_feature);
+end);
+
 local function handle_registration_stanza(event)
 	local session, stanza = event.origin, event.stanza;