Fixed "++" and "+-+" by replacing a*a with sqr(a). Better error message for square root of negative numbers.

Impose mp->stack_size < 1001 in @d push_input to avoid stack overflow due infinite recursion of macro expansion (thanks to A. Kakuto).

Revert the patch in mpx_copy_mpto, it was wrong in tex mode. Added a check if in troff mode. Updated copyright date.