mke2fs: prompt for user verification for "-S"

mke2fs: prompt for user verification for "-S"

Prompt for user verification before rewriting the filesystem
superblocks using the "-S" (super-only) option.  This should
not normally be used at all, so adding the extra verification
will probably save a few user filesystems in the future.  Since
this is something that should only be done in rare cases under
user supervision, wait for user input rather than proceeding
automatically after a timeout.

Update the mke2fs man page to more fully explain the many
dangers of this option.

Signed-off-by: Andreas Dilger <adilger@dilger.ca>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>