udev: complete rework

- systemd puts all into one output now (except for man),
  because I wasn't able to fix all systemd/udev refernces
  for NixOS to work well
- libudev is now by default *copied* into another path,
  which is what most packages will use as build input :-)
- pkgs.udev = [ libudev.out libudev.dev ]; because there are too many
  references that just put `udev` into build inputs (to rewrite them all),
  also this made "${udev}/foo" fail at *evaluation* time
  so it's easier to catch and change to something more specific

pkgs/os-specific/linux/drbd/default.nix

@@ -1,4 +1,4 @@
-{ stdenv, fetchurl, flex, udev, perl }:
+{ stdenv, fetchurl, flex, systemd, perl }:
 
 assert stdenv.isLinux;
 
@@ -18,7 +18,7 @@ stdenv.mkDerivation rec {
 
   preConfigure =
     ''
-      export PATH=${udev.out}/sbin:$PATH
+      export PATH=${systemd.udev.bin}/sbin:$PATH
       substituteInPlace user/Makefile.in --replace /sbin/ $out/sbin/
       substituteInPlace user/legacy/Makefile.in \
         --replace /sbin/ $out/sbin/ \