Add option to configure postscreen in master.cf

There are a couple dozen options for postscreen in main.cf. These are not
handled (yet) but can be set in $extra_main_parameters.
Add options for postscreen pre-greeting tests
Merge pull request #25 from syskill/postscreen

Postscreen
add debian support for spampd
Merge pull request #26 from timogoebel/feature/debian-spampd

add debian support for spampd
Update ChangeLog
add support for multiple directives, including TLS

also SMTP TLS client auth does not require all the sasl options
add TLS client cert paramaters