zstd: use higher level by default and add a max option

zstd: use higher level by default and add a max option

levle 9 is what Arch Linux uses for their packages, so it's a widely
used level which is a good default tradeoff for such images.

Add also zstd-max level, which uses the highest default level which
is still highly efficient on decompression, i.e., while compression
needs more time and resources the decompression doesn't.

Some number for a Debian 11 minimal template from 2021-05-06:

uncomp.    321M    100.0%
gzip       116M     36.1%
zstd  0    106M     33.0%
zstd  9     98M     30.5%
zstd 19     83M     26.8%

So, the still cheap to extract zstd lvl19 need 33M less than the
current default gzip, the access log from our german cdn server show
about 490 successful access to our system images per day, that would
mean that changes saves `490 download/day * 30.4375 days/month * 33M
saved/download` =~ 490 G/month traffic for us and for users.

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>