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

It indicates that script execution exceeded a resource limit (currently only CPU
time limit).

plugins: imap-filter-sieve: cmd-filter-sieve - Avoid using imap_arg_as_nstring() in cmd_filter_sieve_script_parse_value_arg()

The argument can only be a quoted string here, so imap_arg_as_string() will suffice.

This was not supposed to be accepted and led to unexpected behavior:

- FILTER SIEVE PERSONAL NIL was handled as FILTER SIEVE DELIVERY,
- FILTER SIEVE GLOBAL NIL caused an internal error.

Use mail_get_message_id() instead of just mail_get_first_header().

Found by latest clang scan-build.

Nothing changes in the code logic.

The old code seems to have worked correctly already, but now it's a bit
clearer how it was intended to work.

Default is now debug logging, which is used for the IMAP plugins. The LDA Sieve
plugin uses info logging.

When no insertion point was found, it would always insert the new rule at the
beginning, rather than at the end.