Message substitutions would not always cause a proper snapshot.

This is needed for the new extprograms plugin.

Incorporated distinction between original and final envelope recipient in Sieve interpreter, as recently introduced in Dovecot.

- Changed operand read functions to prove exec status return codes in stead of boolean.
- Restructured optional-operand, stringlist and string-match APIs to provide execution status (error code) upon failure.
- Corruption errors are only produced when the binary is actually corrupt. Normal failures should never cause a recompile attempt anymore.
- Simplified optional-operand API.
- Cleaned up test command implementations that use the string-match API.

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.

Body: added test regarding empty bodies and fixed testsuite to flush the message context when the message is changed.