Restructured store action implementation to properly handle an attempt to store (keep) a message in the folder it originates from.

Testsuite: added tests on the envelope of outgoing messages produced by redirect, enotify:mailto, reject and vacation.

Major rework of envelope address handling: a normalized version of the envelope addresses is maintained in the message context and message context is now also available during action execution.

Removed the sieve-filter tool from the default build. Need to specify --with-unfinished-features to get this tool built.

Added compilation support for ereject extension. It is unfinished right now and performs exactly the same action as reject.

Testsuite: added tests for envelope/address test behavior with non-existant subaddress :detail part.

Envelope: fixed bug in application of address parts; failure to obtain the part would cause inappropriate match success (bug reported by Ron Lee)

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.