Part of issue #2077

Fixes #2077.

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.

When we are in single user mode, we still want to have access to
profiles. This way things in Nixpkgs that rely on them getting set
accurately are done in both cases. The point where I hit this is with
using aspell which looks in NIX_PROFILES:

https://github.com/NixOS/nixpkgs/blob/master/pkgs/development/libraries/aspell/default.nix

Before this patch, NIX_PROFILES was never set in single user mode!
This corrects that.

This is now autodetected. There is no need to put it in the profile.

See commit 1bffd83

(cherry picked from commit cbfdea6)

Sync NIX_PROFILES between single-user and multi-user modes

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

…ed unless specified

In `args@{ a ? 1 }: /* ... */` the value `a` won't be a part of `args`
unless it's specified when calling the function, the default value will
be ignored in this case.

My personal point of view is that this behavior is a matter of taste, at
least I was pretty sure that unmatched arguments will be a part of
`args@` while debugging some Nix code last week.

I decided to add a warning to the docs which hopefully reduces the
confusion of further Nix developers who thought the same about `args@`.

…ttern

doc: clarify that optional attrs in a function argument will be ignored unless specified

Turns out we were mis-parsing single-quoted attributes, e.g. 'key2'.

cpptoml now parses almost all examples from the spec.