apply, ls-files: simplify "-z" parsing

apply, ls-files: simplify "-z" parsing

As a short option, we cannot handle negation. Thus a callback
handling "unset" is overkill, and we can just use OPT_SET_INT
instead to handle setting the option.

Anybody who adds "--nul" synonym to this later would need to be
careful not to break "--no-nul", which should mean that lines are
terminated with LF at the end.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>