mkswap, swaplabel: move version number to header

Corrently only the swap version 1 is supported, which is a magic value so
move it to header.

Signed-off-by: Sami Kerola <kerolasa@iki.fi>
mkswap: remove legacy swap structure

If software archeolgists want to know how the old, and unused, swap
header looked they can dig it from the revision history.

Reference: 4c85aa3a4c26f2a2c33bf16960b548d5bbd5b4bf
Signed-off-by: Sami Kerola <kerolasa@iki.fi>
include/swapheader.h: ensure type sizes

Use consistently the same type sizes as in libblkid and kernel.

Signed-off-by: Sami Kerola <kerolasa@iki.fi>
swapon: swaps with legacy version label are not supported

Signed-off-by: Sami Kerola <kerolasa@iki.fi>
swapon, swapheader, mkswap: move swap signature to header

Both swapon and mkswap need to know what is valid device signature, so
share the value.

Signed-off-by: Sami Kerola <kerolasa@iki.fi>