Define stanza's attributes as const char*

In most get-like funcitons libstrophe returns pointer to a string
that resides in an internal structure (e.g. xmpp_stanza_t). Hence,
Profanity must not change such strings. Define respective variables
as 'const char*' to reduce a chance of error and conform future
libstrophe's interface.

This patch mostly replaces 'char *' with 'const char*', but also
fixes two memory leaks after stanza_get_reason(). Add comment within
stanza_get_reason() to fix conflict with different allocator types.

src/muc.c

@@ -723,7 +723,7 @@ muc_jid_autocomplete_add_all(const char *const room, GSList *jids)
         if (chat_room->jid_ac) {
             GSList *curr_jid = jids;
             while (curr_jid) {
-                char *jid = curr_jid->data;
+                const char *jid = curr_jid->data;
                 Jid *jidp = jid_create(jid);
                 if (jidp) {
                     if (jidp->barejid) {