* Implemented Eelco V.'s `nix-env -I' command to specify the default

path of the Nix expression to be used with the import, upgrade, and
  query commands.  For instance,

  $ nix-env -I ~/nixpkgs/pkgs/system/i686-linux.nix

  $ nix-env --query --available   [aka -qa]
  sylpheed-0.9.7
  bison-1.875
  pango-1.2.5
  subversion-0.35.1
  ...

  $ nix-env -i sylpheed

  $ nix-env -u subversion

  There can be only one default at a time.

* If the path to a Nix expression is a symlink, follow the symlink
  prior to resolving relative path references in the expression.

src/nix-env/help.txt

@@ -4,15 +4,16 @@ nix-env [OPTIONS...] [ARGUMENTS...]
 
 Operations:
 
-  --install / -i FILE: add derivations to the user environment
+  --install / -i: add derivations to the user environment
+  --upgrade / -u: upgrade derivation in the user environment
   --uninstall / -e: remove derivations from the user environment
-  --upgrade / -u FILE: upgrade derivation in the user environment
   --query / -q: perform a query on an environment or Nix expression
 
 The previous operations take a list of derivation names.  The special
 name `*' may be used to indicate all derivations.
 
-  --profile / -p [FILE]: switch to specified user environment 
+  --profile / -p [FILE]: switch to specified user environment
+  --import / -I FILE: set default Nix expression
 
   --version: output version information
   --help: display help
@@ -26,10 +27,11 @@ Query types:
 Query sources:
 
   --installed: use installed derivations (default)
-  --available / -f FILE: use derivations available in expression FILE
+  --available / -a: use derivations available in Nix expression
 
 Options:
 
   --link / -l LINK: use symlink LINK instead of (...)/current
+  --file / -f FILE: use Nix expression FILE for installation, etc.
   --verbose / -v: verbose operation (may be repeated)
   --keep-failed / -K: keep temporary directories of failed builds