* lisp.h: Redo flags and XSET slightly to avoid overflow diagnostics.

* lisp.h: Redo flags and XSET slightly to avoid overflow diagnostics.

These changes make compilation easier to follow with Sun cc.
(ARRAY_MARK_FLAG): Make it signed, so that it can be assigned to
EMACS_INT values without provoking overflow diagnostics.
(PSEUDOVECTOR_FLAG): Likewise, for consistency.
(XSET) [! USE_LSB_TAG]: Use unsigned left shift to avoid overflow
diagnostic with signed left shift.