linux: Make Unix domain sockets builtin

This hopefully fixes intermittent initrd failures where udevd cannot
create a Unix domain socket:

  machine# running udev...
  machine# error getting socket: Address family not supported by protocol
  machine# error initializing udev control socket
  machine# error getting socket: Address family not supported by protocol

The "unix" kernel module is supposed to be loaded automatically, and
clearly that works most of the time, but maybe there is a race
somewhere. In any case, no sane person would run a kernel without Unix
domain sockets, so we may as well make it builtin.

http://hydra.nixos.org/build/30001448

nixos/modules/system/boot/kernel.nix

@@ -197,9 +197,6 @@ in
         "hid_generic" "hid_lenovo"
         "hid_apple" "hid_logitech_dj" "hid_lenovo_tpkbd" "hid_roccat"
 
-        # Unix domain sockets (needed by udev).
-        "unix"
-
         # Misc. stuff.
         "pcips2" "atkbd"
 

pkgs/os-specific/linux/kernel/common-config.nix

@@ -36,6 +36,9 @@ with stdenv.lib;
   SCHEDSTATS n
   DETECT_HUNG_TASK y
 
+  # Unix domain sockets.
+  UNIX y
+
   # Power management.
   ${optionalString (versionOlder version "3.19") ''
     PM_RUNTIME y