Remove tabs
initrd: Add ehci_pci and hid_generic

These are required to get some (all?) USB keyboards to work in recent
kernels.
initrd: Remove serio and atkbd

These modules don't exist as far as I can tell.
tests/misc.nix: Test nixos-rebuild --help
Return exit code 127 if a command is not found
Add varnish to module-list.nix
btrfs: btrfsck is now actually the same binary as "btrfs"
symlinked to save space.

Also, removed the hard-coded version-numbers for libz and liblzo.
If they get upgraded and btrfs breaks, either btrfs' build will break,
or the initrd test will anyway.
filesystems: add priority option for swap devices.

Useful for setting up raid0-like load balancing for swap.
By giving multiple swap devices the same prio.
kde4: use udisks2 for kde 4.10+