e2image: add -b and -B options to specify where to find the superblock

e2image: add -b and -B options to specify where to find the superblock

e2image has no ability to use superblock backup to copy metadata.
This feature can be useful if someone wants to make partition
image and fix it using e2fsck utility.

New -b option allows to pass superblock number, like e2fsck utility does.
e2image doesn't change primary superblock and store it as is, so
it can be fixed using e2fsck latter. Option -B allows setting
superblock size.

Signed-off-by: Artem Blagodarenko <c17828@cray.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>