Made vnd.dovecot.debug extension available to the LDA plugin instead of only the command line tools.

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.

- Administrators now get properly notified about uncompiled global scripts and the inability of the sieve plugin to store global binaries.
- Improved binary load/save error handling and fixed a few smal bugs.
- Simplified ManageSieve error handling.

Changed result execution logging to use a var_expand_table created by liblda to avoid code duplication (patch by Timo Sirainen).