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.

lib-sieve: mime extension: Fixed the header :mime :anychild test to work properly outside a foreverypart loop.

lib-sieve: message body: Fixed erroneous changes to the message part tree structure performed when re-parsing the message.

No duplicate check function was implemented, so that the discard action would be duplicated each time it is invoked.
This only affects the action limits (each discard invocation is counted as another new action).
The result of the script execution would be identical.

test suite: mime extension: Improved header test cases to catch header trimming and folding problems.