libcmd: allow setting nested attributes via --arg/--argstr

Closes #496

When running

    nix-build ../nixpkgs --arg config.allowUnfree true -A hello-unfree

the package `hello-unfree` is now built rather than getting an
eval-error rejecting instantiation. This is because `config.allowUnfree`
is now interpreted as nested attribute-set declaration, similar to how
it's done in `nix repl`.

To prevent sudden breakage, this behavior was carefully deprecated with
Nix throwing an error if the identifier for `--arg` is not a pure
identifier, but an expression as above.

Any kind of merging is rejected. I.e. doing

    nix-build ../nixpkgs --arg config '{cudaSupport = true;}' --arg config.allowUnfree true

is prohibited. That way we don't have to think about merge semantics for
cases like this (or even worse `--arg config 'rec { ... }'`). Another
nice side-effect of this is that we don't need to create an EvalState to
force the values and implement merging.

Change-Id: I8b560883a4468a3f32f915764b08f5fdd8fe71bb

doc/manual/src/command-ref/opt-common.md

@@ -177,6 +177,12 @@ Most commands in Lix accept the following command-line options:
   You can override this using `--arg`, e.g., `nix-env --install --attr pkgname --arg system \"i686-freebsd\"`.
   (Note that since the argument is a Nix string literal, you have to escape the quotes.)
 
+  Additionally, dots are interpreted as attribute-path separators.
+  I.e. `nix-instantiate '<nixpkgs>' -A hello-unfree --arg config.allowUnfree true` will result in an argument `config` with value `{ allowUnfree = true; }` being passed to `<nixpkgs>`.
+
+  Please note that merging of different arguments is rejected.
+  I.e. `--arg config '{ cudaSupport = true; }' --arg config.allowUnfree true` will not work whereas `--arg config.cudaSupport true --arg config.allowUnfree true` is accepted.
+
 - <span id="opt-argstr">[`--argstr`](#opt-argstr)</span> *name* *value*
 
   This option is like `--arg`, only the value is not a Nix expression but a string.