* Implemented Eelco V.'s `-p' command to switch profiles. It switches

the symlink ~/.nix-userenv to the given argument (which defaults to
  .../links/current).  /etc/profile.d/nix-profile creates this symlink
  if it doesn't exist yet.  Example use:

  $ nix-env -l my_profile -i foo.nix subversion quake
  $ nix-env -p my_profile

  I don't like the term "profile".  Let's deprecate it :-)

src/nix-env/help.txt

@@ -12,6 +12,8 @@ Operations:
 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 
+
   --version: output version information
   --help: display help