- Implement "dsn" option to listen to activate DSN
Merge branch 'portable' of https://github.com/poolpOrg/OpenSMTPD into dsn_config
 - Change EXT_DSN to DSN
Merge remote-tracking branch 'upstream/portable' into dsn_config
 - Switch from dsn to no-dsn, i.e. : DSN is activate by default
 - Do not parse NOTIFY and ORCPT smtp args if DSN isn't activated
Merge pull request #457 from bocal/dsn_config

Dsn config