The use of a variable substitution erroneously produced a runtime syntax error.

This was broken by recent adjustments to changes in Dovecot lib-storage API.

This allows side-effects to open a different mailbox than what is allocated
initially in act_store_start().

lib-sieve: store action: Allow side-effects to change the mailbox identifier used in action logging.

lib-sieve: plugins: mailbox: tag-mailbox-create - Add variable for trans->box in seff_mailbox_create_pre_execute().

Use mailbox_get_last_error() shorthand rather than
mail_storage_get_last_error().

This way, the mailbox does not need to be opened until result execution.

By setting the MAILBOX_FLAG_POST_SESSION flag, this  makes sure the result of
this test closely matches what would happen for a real delivery action using
"fileinto".

This way, the execution order can be made explicit.

plugins: imap-filter-sieve: Fix mixup between tag and problem part of the FILTER response for compile error.

plugins: imap-filter-sieve: Perform main initialization of Sieve script execution environment only once.

This way, any configuration errors can be reported as a single FILTER ERRORS
response, rather than a FILTERED ERRORS response for each filtered message.

plugins: imap-filter-sieve: Make sure user gets to see at least one error when script execution fails.