Before, actions were executed and committed in full after each script in a
sequence. With this change, actions can still be rolled back until the end of
all Sieve script and action execution.

The committed flag is set when at least one action was committed successfully.
The existing executed flag is repurposed to signal successful execution of at
least one action. These flags are now also properly managed for the keep action.

Allows doing the check in the action finalization function.

It is no longer used.

Adds status parameter to sieve_result_execute().

Makes sure it is consistent for a later commit when action exection is more fragmented.

mailbox_get_last_error() should be used only for client-visible errors.
The internal errors should be used for logging.