Add an ExitRelay option to override ExitPolicy

Add an ExitRelay option to override ExitPolicy

If we're not a relay, we ignore it.

If it's set to 1, we obey ExitPolicy.

If it's set to 0, we force ExitPolicy to 'reject *:*'

And if it's set to auto, then we warn the user if they're running an
exit, and tell them how they can stop running an exit if they didn't
mean to do that.

Fixes ticket 10067