lib-sieve: optimized compilation of tests that yield constant results (i.e. known at compile tme), such as true and false.
If the result of a test is known at compile time, it is optimized away. If an if-command depends on an entirely constant test, it is optimized away as well,
causing only the 'true' sub-block to be compiled.

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.

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

Testsuite: added RFC compliance tests for the variables extension and fixed use of wrong default comparator.

Envelope: <> return path now always matches as the empty string, regardless of the specified address part.

Significantly improved handling of old/corrupt binaries and revised matching implementation in the process.

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

Testsuite: tested handling of teststuite envelope environment and fixed bugs in the envelope test in the process.