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