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.