tar: imply --xattrs when --xattrs-{inc,exc}lude used

tar: imply --xattrs when --xattrs-{inc,exc}lude used

Options --xattrs-include=MASK and --xattrs-exclude=MASK now turn
on the --xattrs option.

Fix also bug in printing in xattrs.c - don't print when option is
negative.

* src/tar.c (set_xattr_option): New static function.
(parse_opt): Call new function when --xatrrs, --xattrs-include or
--xattrs-exclude option is used.
* src/xattrs.c (xattrs_print, xattrs_print_char): Expect positive
values in options.