* Allow extra flags to be passed to RPM.

* Respect DESTDIR variable.

    nix-env -u foo.nix strategoxt

  to replace the installed `strategoxt' derivation with the one from `foo.nix', if 
  the latter has a higher version number.  This is a no-op if `strategoxt' is not 
  installed.  Wildcards are also accepted, so

    nix-env -u foo.nix '*'

  will replace any installed derivation with newer versions from `foo.nix', if 
  available.

  The notion of "version number" is somewhat ad hoc, but should be useful in most 
  cases, as evidenced by the following unit tests for the version comparator:

    TEST("1.0", "2.3", -1);
    TEST("2.1", "2.3", -1);
    TEST("2.3", "2.3", 0);
    TEST("2.5", "2.3", 1);
    TEST("3.1", "2.3", 1);
    TEST("2.3.1", "2.3", 1);
    TEST("2.3.1", "2.3a", 1);
    TEST("2.3pre1", "2.3", -1);
    TEST("2.3pre3", "2.3pre12", -1);
    TEST("2.3a", "2.3c", -1);
    TEST("2.3pre1", "2.3c", -1);
    TEST("2.3pre1", "2.3q", -1);

  (-1 = less, 0 = equal, 1 = greater)

* A new verbosity level `lvlInfo', between `lvlError' and `lvlTalkative'.  This is 
  the default for `nix-env', so without any `-v' flags users should get useful 
  output, e.g.,

$ nix-env -u foo.nix strategoxt
upgrading `strategoxt-0.9.2' to `strategoxt-0.9.3'

    nix-env -i foo.nix subversion

  The version number part of a derivation name is defined as everything following the 
  first dash not followed by a letter.

* `--link' / `-l' flag to specify the switch symlink to use (by default, 
  /nix/var/nix/links/current).

* Integrity: check in successor / substitute registration whether
  the target path exists or has a substitute.

  --register-[substitutes|successors].

$ nix-instantiate --help
error: unknown flag `--help`
Try `nix-instantiate --help' for more information.

:-)

  "i686-linux" instead of "i686-suse-linux").

* Convert tabs to spaces.