fix socketpair leak in 'pony'

On smtp connection exception the socketpair used to i/o the data phase
is not freed. Whenever IMSG_SMTP_MESSAGE_ROLLBACK is signalled to
'queue', 'pony' should release the io resources related to the
socketpair. There are four such exceptions in smtp_io():
IO_{TIMEOUT,DISCONNECTED,ERROR} and SF_BADINPUT.