Compression levels with --merge

Hi Denis,

Thanks again for all the work on dar!  I have a bug report for you.
Following the recipe in the FAQ, I tried to use:

dar -v -+ archive2 -A archive -zzstd:12

Which crashed with:

Aborting program. An error occurred while calling libdar: Compression_level must be between 1 and 9 included

-zzstd:12 works fine in conjunction with -c, as it should (zstd levels
go above 9).

This is with 2.7.17 but I haven't seen anything in the 2.8.1 changelog
to indicate it's fixed there.  Fixing it in the 2.8.x branch would be
fine (I wouldn't see the need for a backported fix to 2.7)

Incidentally, thanks for 2.8.1 and webdar!  I have uploaded 2.8.1 to
Debian unstable and will work on webdar shortly.  Then I will get them
both into trixie-backports as well.

- John