doveadm sieve plugin: Added proper handling of Sieve storage initialization failure occurring when sieve_enabled=no

This allows completely disabling Sieve processing for a particular user.

It now extracts bare text from HTML/XHMTL parts.
Other text/* parts are still returned as is. Any other unrecognized content types are skipped.

This allows setting the MAIL FROM of the SMTP envelope for the notification e-mails.
The syntax is identical to the sieve_redirect_envelope_from setting.

lib-sieve: Created generic implementation for parsing an envelope_from setting, such as sieve_redirect_envelope_from.
Also adds a new source for the envelope_from address called "postmaster". This means that the value is obtained from the postmaster_address LDA setting.

lib-sieve: script storage: Sieve mailbox attributes use an external storage, so they don't need to be synced explicitly with any mail storage backend.
Patch by Timo Sirainen.

doveadm sieve plugin: Fixed crashes caused by incorrect context allocation in the command implementations.
The base context struct was allocated, rather than the (larger) command-specific context.
Patch by Timo Sirainen.

This makes sure that the resulting mail storage events are of the correct type for logging/notification.

This causes link failures for certain platforms.

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.