Improve MUC 1:1 logging

If we get a private message from a user in a MUC profanity shows this
like:
`profanity@roomsASDF.dismail.de/Martin: Hi`

This was so far logged at:
`~/.local/share/profanity/chatlogs/my-account-at-server/profanity_at_rooms.dismail.de/2019_09_04.log` as:

```
10:48:13 - profanity@rooms.dismail.de: Hi
```

So the nickname was not saved anywhere. This is due to us not knowing
whether we got a regular message from user@server.org/resource or a MUC
PM from room@server.org/user.

We now check for `<x xmlns='http://jabber.org/protocol/muc#user' />` and
add the resourcepart to the logging if we get it.

The file will be created at
`~/.local/share/profanity/chatlogs/my-account-at-server/profanity_at_rooms.dismail.de_nick` and look like:

```
23:59:43 - nick: Hi
```

Fix https://github.com/profanity-im/profanity/issues/1184

src/xmpp/message.h

@@ -56,6 +56,7 @@ typedef struct prof_message_t {
    GDateTime *timestamp;
    prof_enc_t enc;
    gboolean trusted;
+   gboolean mucuser;
 } ProfMessage;
 
 typedef int(*ProfMessageCallback)(xmpp_stanza_t *const stanza, void *const userdata);