mkfs: move shared config structs and into their own headers

mkfs: move shared config structs and into their own headers

Both struct sb_feat_args and struct mkfs_default_params will be shared
between CLI processing and the configuration file processing added later,
so move these to their own header.

This will help ensure we split things neatly later and also will help
ensure the configuration file processing code from the CLI code are kept
separate and cannot touch each other's data structures. This also makes
it clear what is actually shared between both.

There are no introduced functional changes in this commit and no
documentation changes, this is just code shuffling.

Signed-off-by: Luis R. Rodriguez <mcgrof@kernel.org>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>