Added support for doing routing at the MTA, that is the right thing

Added support for doing routing at the MTA, that is the right thing
anyway.  A new syntax is defined, "present", that indicates that
values of an attribute type are not used, only presence is
significant.  To do routing at the MTA, define both mailHost and
mailRoutingAddress with syntax "present".  Otherwise, use "host" and
"rfc822" and mail500 will try to do routing by itself, if possible.
Read the comments in the code for the ugly details.
Added a new configuration line "own-address" that describes the FQDN of
our host to compare with mailHost.  The line can be repeated.