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/event/server_events.c

@@ -508,7 +508,7 @@ sv_ev_incoming_carbon(char *barejid, char *resource, char *message, char *pgp_me
 }
 
 void
-sv_ev_message_receipt(char *barejid, char *id)
+sv_ev_message_receipt(const char *const barejid, const char *const id)
 {
     ProfChatWin *chatwin = wins_get_chat(barejid);
     if (!chatwin)