configitems: register server.zstdlevel
Somehow, I managed to trigger a devel-warn running `hg serve` outside the test
suite on one of the repos generated by running test-lfs-serve-access.t --keep.
(I'm not hitting it now after doing a `make local`.) The only reference to this
in all of the history is the help text added in e75463e3179f, and the
translations. (It looks like the string is built dynamically with '%slevel',
which is probably how this was missed. I wonder if this isn't getting routed to
error.log for some reason.)
In any event, server.zliblevel is registered, and that's required to pick up the
documented default.