mod_antispam: initial import from ejabberd-contrib/mod_spam_filter

[sstrigler]

At that point this is very preliminary. Missing

• tests
  Not sure where to add tests. Would be a major but rewarding effort. Please advice!
• documentation
  Add documentation for mod_antispam docs.ejabberd.im#136 - for current status
• merge mod_muc_rtbl (maybe)
  Otherwise we'd have to configure RTBL hosts in different places. Also code-wise we would avoid duplicate functionality and avoid differing behavior

Current config:

modules:
 [...]
  mod_antispam:
    rtbl_host: "xmppbl.org"
    rtbl_domains_node: "spam_source_domains"
    spam_domains_file: "/etc/ejabberd/spam-filter/domains.txt"
    spam_jids_file: "/etc/ejabberd/spam-filter/jids.txt"
    spam_urls_file: "/etc/ejabberd/spam-filter/urls.txt"
    spam_dump_file: "/var/log/ejabberd/spam_dump.log"
    access_spam:
      allow:
        user: honeypot@example.org

If mod_muc_rtbl's functionality were to be merged into this module that config might need to change, especially wrt either have just one or more rtbl hosts.

Proposed change:

modules:
  [...]
  mod_antispam:
    rtbl_hosts:
      - xmppbl.org:
        - spam_source_domains
        - muc_bans_sha256
      - rtbl.example.org:
        - spam_source_domains: custom_node
    rtbl_policy:
      spam_source_domains: more_than_one
      muc_bans_sha256: one

Possible rtbl_policy values are one|more_than_one|all. Ideas?

[sstrigler]

I had plans to remove the gen_server in this code and instead just use ets tables. Would make the code easier to navigate since you wouldn't have to jump between different places. But then, keeping the fd for the dump file requires a gen_server, that's why I stuck with it for now.

[coveralls]

coverage: 33.238% (-0.3%) from 33.58%
when pulling 94d008a on sstrigler:mod_antispam
into ef75493 on processone:master.