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.