nix-env -i: Add a flag ‘--remove-all’ / ‘-r’

This is equivalent to running ‘nix-env -e '*'’ first, except that it
happens in a single transaction.  Thus, ‘nix-env -i pkgs...’ replaces
the profile with the specified set of packages.

The main motivation is to support declarative package management
(similar to environment.systemPackages in NixOS).  That is, if you
have a specification ‘profile.nix’ like this:

  with import <nixpkgs> {};
  [ thunderbird
    geeqie
    ...
  ]

then after any change to ‘profile.nix’, you can run:

  $ nix-env -f profile.nix -ir

to update the profile to match the specification.  (Without the ‘-r’
flag, if you remove a package from ‘profile.nix’, it won't be removed
from the actual profile.)

Suggested by @zefhemel.