Mimics similar change in Dovecot.

Also made sure that any capability-derived state information is reset properly before a new capability reply is to be parsed.

lib-sieve: multiscript: Fixed bug in handling of (implicit) keep; final keep action was executed as though there was a failure.
Among other things, this caused the keep action to revert back to the initial message, causing editheader actions to be ignored.

Addresses LLVM scan-build report.

lib-sieve: storage: Local variable in sieve_storage_active_script_is_default() wasn't always properly initialized.
This was a LLVM scan-build report.

lib-sieve: imap4flags extension: Similar problem as previous commit, but relating to retrieval rather than manipulation.

Flags weren't always assigned properly, because the internal variable was not retrieved correctly.

lib-sieve: imap4flags extension: Forgot to check for empty flag/keyword in new flag checking function.

Added support for getting message part information and content.

Prevents useless jumps after break operation.

lib-sieve: Added support for having multiple instances of a single tag (type) within a single command.

Would not occur in Pigeonhole itself.