Add OPAMROOT environment variable.

This enables:
* 'opam --root $dir config -env' to output the right $OPAMROOT variable;
* which means subsequents calls to 'opam' will use the right root path.

This help installing opam on multi-user computers (where root is set to /usr/lib/opam for instance).

This should fix ocaml#302

src/client/opamMain.ml

@@ -28,8 +28,6 @@ let bad_argument cmd fmt =
 let noanon cmd s =
   raise (Bad (cmd, s ^ " is not expected"))
 
-let () = OpamGlobals.root_dir := OpamGlobals.default_opam_dir
-
 (* Useful for switch, which can overwrite the default verbose flag *)
 let quiet = ref false
 

src/client/opamState.ml

@@ -552,6 +552,13 @@ let get_env t =
   let add_to_env = expand_env t add_to_env in
   let new_env = expand_env t new_env in
 
+  (* if --root <dir> is passed on the command line, or if OPAMROOT is set. *)
+  let new_env =
+    if !OpamGlobals.root_dir <> OpamGlobals.default_opam_dir then
+      ("OPAMROOT", !OpamGlobals.root_dir) :: new_env
+    else
+      new_env in
+
   { add_to_env; add_to_path; new_env }
 
 let print_env_warning ?(add_profile = false) t =

src/core/opamGlobals.ml

@@ -42,12 +42,16 @@ let switch : string option ref = ref None
 let opam_version = "1"
 
 let home =
-  try Unix.getenv "HOME"
-  with _ -> Unix.getcwd ()
+  try Sys.getenv "HOME"
+  with _ -> Sys.getcwd ()
 
-let default_opam_dir = Filename.concat home ".opam"
+let default_opam_dir =
+  Filename.concat home ".opam"
 
-let root_dir = ref default_opam_dir
+let root_dir = ref (
+  try Sys.getenv "OPAMROOT"
+  with _ -> default_opam_dir
+)
 
 let log section fmt =
   Printf.ksprintf (fun str ->