Implement table-redis backend

Does not support the table_fetch command.

Conflicts:
	configure.ac
	mk/backends/Makefile.am
Fix a bug and add a config example
Add Makefiles for OpenBSD version of table-redis
Remove #include
Merge pull request #397 from mneumann/master

Bring in table-redis for master branch
Implement a mechanism by which the admin can forbid the mta to
establish connections from a particular ip address for a given
destination domain. New smtpctl commands:

  smtpctl pause mta from <addr> for <domain>
  smtpctl resume mta from <addr> for <domain>
  smtpctl show mta paused
Merge branch 'master' into portable

Conflicts:
	smtpd/table_redis.c