lib-sieve: vacation extension: finally added support for using the original recipient in vacation address check.

- 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.

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

Added the concept of persistent tags and implemented imapflags extension for bare keep and fileinto commands (intermittent commit, not working properly yet).