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.

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

Working towards improving the handling of currupt binaries: defined multiple exit codes for execution functions and defined trace macro for reporting binary corruptions.