Do not process presence updates as GC1.0 joins

I had an IRC channel MUC history duplication issue once again. Seems like yaxim sent a presence update in the morning, some time after the IRC server was restarted, and biboumi treated it as a join:

2019-11-12_07:05:57.95514 [DEBUG]: src/xmpp/xmpp_component.cpp:162:     XMPP RECEIVING:
<presence from="georg@domain/yaxim" id="F6PA5-62647" to="#channel%network@irc.domain/nickname">
  <status />
  <priority>-10</priority>
  <c xmlns="http://jabber.org/protocol/caps" hash="sha-1" node="https://yaxim.org/" ver="PNyKvAFdHcM4HyMVijywvPee9+o="/>
</presence>

Biboumi treated that as a join request and responded with a full MUC history. Due to #3392, it was not possible to deduplicate that, leading to doubled lines in the log.

GC1.0 has been deprecated, so please treat this as a presence update instead, and return an error if the presence is not from a room occupant. The recommended error condition is not-acceptable.

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