Log Message:
Update net/erlang-xmpp to 1.1.17.

- Register pubsub#multi-items feature and add pubsub#max_items
  field to meta-data
- Mention try_subtag/2 in the README
- Fix formatting in API.md
- New function: xmpp:try_subtag/2
- Make pretty printer understand jid records
- Support XEP-0184: Message Delivery Receipts
- Improve pretty printer and sub-elements processing
- Add PubSub error condition: 'precondition-not-met'
- pubsub_publish_options.xdata: Reject 'secret'
- Allow empty URIs in xmpp_uri:check/1
- Update muc#roominfo form and add validation for
  'muc#room*_pubsub' fields
- Introduce xmpp_uri module