lib-sieve: message body: Fixed assert failure occurring when text extraction is attempted on a empty or broken text part.

lib-sieve: message body: Explicitly prevent attempting text extraction from header and multipart body parts.

This error can apparently occur if the program terminates earlier than the program client issues shutdown().

No final '\0' character was added to those body parts.
This occurs for the body test with the (default) `:text' body transform.

lib-sieve: code: Added assert to prevent emitting invalid byte code when extension definitions are wrong.

Also improved handling of permission errors caused by ACL plugin.

lib-sieve: Fixed handling of global errors. If master and user error handler were identical, in some cases the log message could be lost.

lib-sieve: result: Fixed handling of implicit keep for a partially executed transaction that yield a temporary failure.
It would probably result in message duplication.

LDA Sieve plugin: Made sure there is always a proper error handler for the implicit keep action during multiscript execution.
Should fix reported bug on missing message-ID in log.

This would probably cause a segfault at multiscript errors.

Should not dereference array_idx() result on the spot.

Error code was not passed back to caller, so that it operates on an uninitialized value.
Bug found by CLang warning.

It is now named pg_t_str_trim() to prevent a new conflict at the next attempt.

Included scripts now execute in the context of the inner parent foreverypart loop.

An empty string yielded an empty string rather than "0".

The free handler of code dumper extensions was never executed.

Needed for test suite.

When the program crosses the boundary of the current jump somehow, the program is terminated and a corruption error is returned.

lib-sieve: mime/foreverypart: Fixed code dumping of `:param' argument. It erroneously reported a code corruption.

lib-sieve: Created special stringlist class for message headers, so that header names can also be returned by the iterator.

lib-sieve: Moved handling of implicit keep during multiscript execution outside the script sequence itself.
Before, implicit keep was executed as part of the final script execution. This caused the implicit keep to be executed as part of that script, including global context.
This caused insignificant errors during delivery to be logged as errors in the administrator log when the last script is global.