The --quiet and -v/--verbose flags were removed during the glog
conversion in d0c44425e1, which has
previously broken programs like e.g. home-manager, which passes --quiet
to nix-build.
A nix-build-specific workaround was added in
24f9354d5b, which manipulates the
FLAGS_stderrthreshold global variable from glog. This commit moves the
--quiet logic back into the argument handling code in libmain, and adds
corresponding handling for -v/--verbose.
Change-Id: I13d860ebbb78541d9f1236691a1efe8bd2163c67
Reviewed-on: https://cl.tvl.fyi/c/depot/+/2170
Tested-by: BuildkiteCI
Reviewed-by: glittershark <grfn@gws.fyi>