(enum): Add UNLINK_DEST_BEFORE_OPENING.

(enum): Add UNLINK_DEST_BEFORE_OPENING.
[long_options]: Add an entry for --remove-destination.
(usage): Describe --remove-destination.
(do_copy): Use unlink_dest_after_failed_open member, not `force.'
(cp_option_init): Initialize new members.
(main): Handle UNLINK_DEST_BEFORE_OPENING (aka --remove-destination).