systemd: Separate lib output

This moves libsystemd.so and libudev.so into systemd.lib, and gets rid
of libudev (which just contained a copy of libudev.so and the udev
headers). It thus reduces the closure size of all packages that
(indirectly) depend on libsystemd, of which there are quite a few (for
instance, PulseAudio and dbus). For example, it reduces the closure of
Blender from 430.8 to 400.8 MiB.

pkgs/os-specific/linux/multipath-tools/default.nix

@@ -1,4 +1,4 @@
-{ stdenv, fetchurl, lvm2, libaio, gzip, readline, udev }:
+{ stdenv, fetchurl, lvm2, libaio, gzip, readline, systemd }:
 
 stdenv.mkDerivation rec {
   name = "multipath-tools-0.5.0";
@@ -19,7 +19,7 @@ stdenv.mkDerivation rec {
   '';
 
   nativeBuildInputs = [ gzip ];
-  buildInputs = [ udev lvm2 libaio readline ];
+  buildInputs = [ systemd lvm2 libaio readline ];
 
   makeFlags = [
     "LIB=lib"