* 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/libutil/util.cc

@@ -105,8 +105,7 @@ Path canonPath(const Path & path, bool resolveSymlinks)
             /* If s points to a symlink, resolve it and restart (since
                the symlink target might contain new symlinks). */
             if (resolveSymlinks && isLink(s)) {
-                followCount++;
-                if (followCount >= maxFollow)
+                if (++followCount >= maxFollow)
                     throw Error(format("infinite symlink recursion in path `%1%'") % path);
                 temp = absPath(readLink(s), dirOf(s))
                     + string(i, end);