implement fs-lib, squash a few bugs that were part of det_fs/wrap_fsck

implement fs-lib, squash a few bugs that were part of det_fs/wrap_fsck

To not pollute dracut-lib.sh, all the fsck related functions were moved
to fs-lib.sh. The functions available are as follows:

- fsck_single

this will detect/verify filesystem, check if it has necessary tools and
check the filesystem respecting additional flags (if any), using
specific "driver" (or falling back to generic one). Currently
available: fsck_drv_{com,xfs,std}. 'com' is used for tools following
typical subset of options/return codes (e.g. ext, jfs), 'std' is used
for "unknown" fs and doesn't assume it can be run non-interactively.

Please see comments around the code for more info.

- fsck_batch

this will check provided list of the devices;

Both of the above functions will fake empty fstab, to make generic fsck
not complain too much (excact devices are always provided on the command
line).

"Known" filesystems currently: ext234, reiser, jfs, xfs

- det_fs

Small bug fixed - as this function is meant to be called in $(), it may
not be verbose.
Current behaviour is:
 - if detection is successful, use its result
 - if detection is not successful, and filesystem is provided, return
   the provided one; otherwise use auto