Update chat/ejabberd to 17.12.

pkgsrc
- Change installation home for Erlang libs to prevent build conflicts
  with previous versions that may be present on the system, as
  encountered by @gdt.

Core
- Rewrite ejabberd system monitor for efficiency
- Fix incoming XMPP processing order
- Use xmpp:try_subtag/2 wherever possible
- Better process subtag decoding errors
- Only allow compression after SASL as per XEP-0170
- Don't crash on unexpected XML events
- Fix session mnesia table cleanup
- Don't let privacy list prevent local roster update

Encryption
- Rely on Server Name Indication for incoming Direct-TLS connections
- Speedup certificate chains creation and validation
- Log warning on empty wildcard paths
- Don't call pkix_is_self_signed/1 too frequently
- Eat less memory during building certificates graph
- Avoid infinite loop between self-signed certs
- Fix function clause on filelib:wildcard/1
- Use ejabberd_pkix API in mod_sip
- Move 'certfile' based options in a single place

Groupchat
- Don't crash on malformed IQ
- Include x tag in presence errors related to nick change
- Include 110 status on shutdown
- Improve muc#roominfo and muc#roomconfig forms
- mod_mam: Never store MUC messages in user archives

PubSub
- Fix num_subscribers on node metadata
- Fix send last items on initial presence
- Send last PEP items to owner on initial presence
- Support pubsub#publish-options PRECONDITIONs
- Add pubsub#multi-items to features list (Support XEP-0060 v1.14)
- Add missing Nidx building records from sql result
- Don't force RSM in get_items when max_items is not provided

Admin
- Introduce option 'ca_file'
- Set executable permission on mac_listener when installing
- Use /bin/sh as the explicit shell when using su in ejabberdctl.
- Windows does not have /tmp, fallback to $HOME/conf for ODBC
  configuration files

Logging
- Change loglevel of TLS failures
- Log a warning when a disk is almost full
- Disable default alarm handler
- Handle also process_memory_high_watermark alarm
- Kill and restart lager when it's overloaded
- Avoid excessive logging of SQL failures

Commands
- New muc_online_room_by_regex command
- Fix race between join_cluster and ejabberd_mnesia
- Fix commands rooms_unused_list and _destroy
- send_direct_invitations accepts only user jids
- ejabberd_sm: Fix get_session_sid/3

SQL
- Add missing server_host column in pg.new.sql
- Add new schema for MySQL and Sqlite
- Fix SQL serialization

Installer
- MacOS installer is signed. You can now easily install ejabberd on
  your Mac.

Build
- Compile sql_pt early
- Binary installer uses OTP 20.2

(fhajny)