Added ability to assume FQDNs for remote hosts on relays

When you setup a relay and remote hosts using it don't specify an FQDN in the from address, mail will bounce when  it reaches most MTAs.  To combat this, the append_dot_mydomain setting should be toggled to yes and the remote_header_rewrite_domain option should be used.  The myorigin option only applies to email relayed from localhost.  Mail relayed from remote hosts uses remote_header_rewrite_domain.