* nix-env -e: support uninstalling by path, so that one can say

$ nix-env -e $(which firefox)

  or

    $ nix-env -e /nix/store/nywzlygrkfcgz7dfmhm5xixlx1l0m60v-pan-0.132

* nix-env -i: if an argument contains a slash anywhere, treat it as a
  path and follow it through symlinks into the Nix store.  This allows
  things like

    $ nix-build -A firefox
    $ nix-env -i ./result

* nix-env -q/-i/-e: don't complain when the `*' selector doesn't match
  anything.  In particular, `nix-env -q \*' doesn't fail anymore on an
  empty profile.

src/libstore/store-api.hh

@@ -174,11 +174,21 @@ bool isStorePath(const Path & path);
 
 void checkStoreName(const string & name);
 
+
 /* Chop off the parts after the top-level store name, e.g.,
    /nix/store/abcd-foo/bar => /nix/store/abcd-foo. */
 Path toStorePath(const Path & path);
 
 
+/* Follow symlinks until we end up with a path in the Nix store. */
+Path followLinksToStore(const Path & path);
+
+
+/* Same as followLinksToStore(), but apply toStorePath() to the
+   result. */
+Path followLinksToStorePath(const Path & path);
+
+
 /* Constructs a unique store path name. */
 Path makeStorePath(const string & type,
     const Hash & hash, const string & suffix);