Quote variables where word splitting is clearly not indended.

Same wording, more readable.

Add support for some options at install time:

  --channel=URL
          The Nix channel to get expressions from.
          If URL is empty, no channel will be registered.
          Default: https://nixos.org/channels/nixpkgs-unstable

  --channel-name=NAME
          Optional name to give the Nix channel
          Default: nixpkgs

Also add a bit of help text, invoked with --help.

There is already code in the Nix installer script to subscribe the user
to a channel, but the code is bypassed when nix-profile.sh (which is
sourced by the installer) has already subscribed the user to a channel.

So instead of having two different code paths for doing mostly the same,
this patch leaves only one.

Another reason for removing this code is that if the user runs
"nix-channel --remove nixpkgs", with the intention to build from source
(or for whatever reason), on the next login the user is again subscribed
to nixpkgs channel. That seems like an anti-feature to me.

Update "tests/nix-profile.sh" to reflect that ~/.nix-channels is no
longer created by this script.