fsck.cramfs: refactor option -x/--extract

fsck.cramfs: refactor option -x/--extract

We want to clean it up to compile it later per default (not only
if defined INCLUDE_FS_TESTS).

- Rename option --destination to --extract.
- DIR argument is now optional because we want to keep the use
  case "test uncompression without writing files" ... but we
  don't want it always.
- Remove shortopt -x because shortopts with optional args are
  evil.

Changing the cmd interface is no compatibility issue here because
all affected options errored out per default in past.

Signed-off-by: Ruediger Meier <ruediger.meier@ga-group.nl>