Send stream trailer before closing s2s connection

When aborting an incoming s2s connection due to certificate verification
errors, send a stream trailer before closing the socket.
Don't mess with s2s out when aborting s2s in

Don't try to look up and close outgoing connections to a given server
when aborting incoming connections from that server due to certificate
verification errors.  The ejabberd_s2s:find_connection/2 call actually
created one or more *new* connections if less than 'max_s2s_connections'
connections were found.  Then, no more than one of those possibly new
connections were stopped by the ejabberd_s2s_out:stop_connection/1 call.

It's not really necessary to bother with outgoing connections at all,
here.
Merge pull request #184 from weiss/properly-abort-s2s-in

Fix handling of certificate verification errors for incoming s2s connections