Made deprecated notify extension implementation compatible with cmusieve, except for the denotify command.

Fixed validator extension validation. It validated the first non-require command before validating the extensions, which produced useless error messages.

Enhanced extensions configuration, allowing to specify the enabled extensions relatively to the default.

Improved error message for unknown Sieve extension to account for core commands included as an extension.

Fixed bug in the Sieve plugin's return value that caused omission of delivery when no Sieve scripts are present. Bug spotted by Matthijs Kooijman.