Incorporated distinction between original and final envelope recipient in Sieve interpreter, as recently introduced in Dovecot.

Sieve plugin: only refer to user log for compile error details when there is actually a compile error.

Produce a nicer error message when trying to load with non-lda/lmtp binary (patch by Timo Sirainen).

- 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.