Log Message:
Changes 0.4.9:
* Properly implemented checking of ABI version for Sieve interpreter
  plugins, much like Dovecot itself does for plugins. This will prevent
  plugin ABI mismatches.
+ Implemented a vnd.dovecot.environment extension. This builds upon the
  standard environment extension and adds a few more environment items,
  such as username and default mailbox. It also creates a variables
  namespace so that environment items can be accessed directly. I am
  still thinking about more environment items that can be added.
+ Sieve extprograms plugin: Made line endings of the input passed to the
  external programs configurable. This can be configured separately for
  each of the three extensions.
+ ManageSieve: Implemented proxy XCLIENT support. This allows the proxy
  to pass client information to the back-end.
- ManageSieve: Fixed an assert failure occurring when a client
  disconnects during the GETSCRIPT command.
- doveadm sieve plugin: Fixed incorrect initialization of mail user.
  This caused a few memory leaks.
- sieve-filter command line tool: Fixed handling of failure-related
  implicit keep when there is an explicit default destination folder.
  This caused message duplication.
- lib-sieve: Fixed bug in RFC5322 header folding. Words longer than the
  optimal line length caused empty lines in the output, which would
  break the resulting message header. This surfaced in References:
  headers with very long message IDs.