resurrect dsn fields
dump/load dsn fields of the envelope
ignore obj directories
MAIL and RCPT commands now understand optional dsn parameters
dump dsn paramters while dumping envelope, missed in earlier commit
Defer orcpt for now, sendmail command line doesn't have a flag to
specify it.
Accept sendmail compatible commandline args for dsn parameters while enqueue
Get rid of a struct and make dsn parameters first class members of
envelope structure.

Introduce new delivery_type for dsn.
Merge branch 'dsn' of https://github.com/snimmagadda/OpenSMTPD into dsn