For now it just sets the internal error. It would be better to change it to
use the actual error though.

This fixes using e.g. %{user} variable in sieve settings.

plugins: lda-sieve: lda-sieve-plugin - Make sure context structure does not retain dangling pointers

Based on report by scan-build.

plugins: imapsieve: imap-sieve - Make sure imap_sieve_handle_exec_status() is not called with rscript == NULL

Based on report by scan-build.

Since the script storage code is widely engrained in the Pigeonhole sources,
this is a very large commit that cannot be reduced further into smaller
preparatory bits.

Later, this is needed to allow retrieving a global script from one of many
storages, rather than just one.

For now, these yield the same result.

Used user event before. This will be needed for retrieving settings in mailbox
context.

plugins: imapsieve: imap-sieve-settings - Properly check imapsieve_url setting for Sieve URL validity

It doesn't belong there.

This makes the extension of the registered item available at evaluation time,
which is e.g. needed to access the extension context.

plugins: doveadm-sieve: doveadm-sieve-sync - Refactor error handling in sieve_attribute_set_active()

Was handled in each extension before, which is useless.