Log Message:
Update chat/ejabberd to 17.09.

Admin
- Harden ejabberdctl
- Fix ejabberdctl quoting when using iex
- Call earlier deps configure scripts during compilation
- Fix iexdebug and iexlive commands
- Quote $PEER in ping command to avoid hostnames containing "-" being
  interpreted as arithmetic
- Docker: Sync containers from rroemhild and add instructions in
  README
- Use eimp instead of ImageMagick calls for thumbnails creation
- Add forgotten caching options to the validator
- Fix 'make install' to work with new output from rebar list-deps
- Rewrite muc_register_nick and muc_unregister_nick to be DB
  independent
- WebAdmin: Fix deletion of multiple offline messages

Encryption
- Add support for XEP-0368 in outgoing s2s: SRV records for XMPP over
  TLS
- Deprecate s2s_use_starttls: required_trusted
- Don't attempt to access(2) a certificate file
- Let 'domain_certfile' take higher precedence instead of s2s_certfile
  or c2s_certfile

Databases
- mysql.sql: Use multi-column index on username/ID
- Use forked repo of Riak Erlang client to support OTP20

Modules
- mod_avatar: New module with support for legacy and modern clients
- mod_block_strangers: Introduce option 'allow_transports'
- mod_block_strangers: Block messages from strangers before
  mod_mam/mod_offline processing
- mod_http_upload: Don't ignore 'custom_headers'
- mod_muc: Improve presence-error and unavailable of multi-session
  occupants
- mod_multicast: Fix start and reading of configured limits
- mod_mam: Simplify check for anon MUC JID filtering
- mod_mam: Refuse filtering anon MUC queries by JID
- mod_privacy: Explicitly match against
- mod_register: Introduce 'redirect_url' option
- mod_stream_mgmt: Delete 'c2s_init' hook
- mod_vcard_xupdate: Also replace vcard-x-update in direct presences

PubSub
- Fix get_items/get_item calls
- Add correct order when requesting all items
- Implement '6.5.7 Requesting the Most Recent Items'
- Fix RSM support on SQL
- Add RSM support on mnesia
- Fix node_options: default options only apply on first plugin
- Broadcast updated node configuration
- Enforce controls on publish and delete items

Miscelanea
- Preserve correct order of deserialized XML elements
- Suppress push notifications for online clients
- Extract strings and prepare translation files works again