* src/groff/preproc/pic/lex.cpp (table): Add box attributes

* src/groff/preproc/pic/lex.cpp (table): Add box attributes
`xslanted' and `yslanted'.

* src/groff/preproc/pic/object.cpp (object_spec::object_spec):
Initialize `xslanted' and `yslanted'.
(graphic_object): Add methods `set_xlanted' and `set_yslanted'.
(closed_object): Ditto.
Add members `xslanted' and `yslanted'.
(box_object::print): Use them.
(object_spec::make_object): Handle slant values.
 * src/groff/preproc/pic/object.h (IS_XSLANTED, IS_YSLANTED): New
constants.
(object_spec): Add members `xslanted' and `yslanted'.
* src/groff/preproc/pic/pic.y (XSLANTED, YSLANTED): New left-valued
tokens.
Add rules for them.

* src/groff/preproc/pic/pic.man, NEWS, doc/pic.ms: Document above
changes.