Add ability to disable avatar publishing

Add "/avatar disable" to comply with point "3.5 Publisher Disables
Avatar Publishing" in XEP-0084.

src/command/cmd_defs.c:2416
Add "disable" argument. Reword the "/avatar" command description
so it flows better.

src/command/cmd_ac.c:1101
Add "disable" to the "/avatar" autocomplete dictionary.

src/command/cmd_funcs.c:9277
Split "/avatar" commands into two groups with an if statement; those
with a parameter and those without. "cons_bad_cmd_usage()" is in both
groups, which is messy. "disable" has similar logic to "set", but it
includes a failure message.

src/xmpp/avatar.c:152
"avatar_publishing_disable()" uses the same logic to publish metadata
as in "avatar_set()".

src/xmpp/avatar.c:238
Add a message to inform users when they do not receive an avatar after
using "/avatar get" and "/avatar open". In case of a failure, the user
will be subscribed to future avatar updates as long as they continue
to use their current instance of profanity. Adding
"caps_remove_feature()" after "cons_show()" will unsubscribe the user
from avatar updates and prevent the user from downloading an avatar
unexpectedly hours later without issuing an "/avatar" command.

src/xmpp/stanza.c:2698
The new "disable" function follows the same logic as
"stanza_create_avatar_metadata_publish_iq()".

src/xmpp/avatar.c

@@ -150,6 +150,17 @@ avatar_set(const char* path)
 }
 #endif
 
+gboolean
+avatar_publishing_disable()
+{
+    xmpp_ctx_t* const ctx = connection_get_ctx();
+    xmpp_stanza_t* iq = stanza_disable_avatar_publish_iq(ctx);
+    iq_send_stanza(iq);
+    xmpp_stanza_release(iq);
+
+    return TRUE;
+}
+
 gboolean
 avatar_get_by_nick(const char* nick, gboolean open)
 {
@@ -224,6 +235,9 @@ _avatar_metadata_handler(xmpp_stanza_t* const stanza, void* const userdata)
                         _avatar_request_item_by_id(from, data);
                     }
                 }
+            } else {
+                cons_show("We couldn't get the user's avatar, possibly because they haven't set one or have disabled avatar publishing. "
+                          "During this Profanity session, you will receive future changes to this user's avatar.");
             }
         }
     }