Sieve extprograms plugin: Made line endings configurable for the input passed to the external program.

Passed NULL as reason to client_disconnect(), which is not allowed.

lib-sieve: variables extension: Added asserts to extension API to make sure variables extension instance gets passed as argument.

This adds the vnd.dovecot.default-mailbox and vnd.dovecot.username environment items.
It creates the 'env' variables namespace through which all environment items can be accessed directly.
This extension is yet to be documented properly.
More environment items are under consideration.

sieve-filter: Fixed handling of failure-related implicit keep when there is an explicit default destination folder.
The message was left in the source mailbox, even though it was stored in the destination folder.

Previous change had several problems.

It now uses dsasl-client, so it should now be possible to use SASL mechanisms other than PLAIN.
Cleaned up the code a bit.