lib-sieve: store action: copy flags and keywords from input mail, also when imap4flags extension is not active.
This is currently only relevant for sieve-filter, which now properly preserves flags with this change.

It did not use the original message in this case.

- 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.

Fixed logging of mailbox names. It logged the converted mUTF7 version in stead of the original UTF8.

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