Sometimes, "expected" can be "0", but in fact means "unknown".

This is for example the case when downloading a file while the http
server doesn't send the `Content-Length` header, like when running `nix
build` pointing to a nixpkgs checkout streamed from GitHub:

⇒  nix build -f https://github.com/NixOS/nixpkgs/archive/master.tar.gz hello
[1.8/0.0 MiB DL] downloading 'https://github.com/NixOS/nixpkgs/archive/master.tar.gz'

In that case, don't show that weird progress bar, but only the (slowly
increasing) downloaded size ("done").

⇒  nix build -f https://github.com/NixOS/nixpkgs/archive/master.tar.gz hello
[1.8 MiB DL] downloading 'https://github.com/NixOS/nixpkgs/archive/master.tar.gz'

This commit also updates fmt calls with three numbers (when something is
currently 'running' too) - I'm not sure if this can be provoked, but
showing "0" as expected doesn't make any sense, as we're obviously doing
more than nothing.

progress-bar: hide expected if expected is 0 (unknown)