cobol: Refactor the gmath.cc "int256" structure.

The int256 structure is used for doing fixed-point arithmetic.  This
rewrite incorporates "int rdigits" into the structure, so that the number
of digits to the right of the decimal point is carried as part of the
structure instead of being carried by external logic.

gcc/cobol/ChangeLog:

	* copybook.h (_COPYBOOK_H): #include <sys/types.h> for macOS.
	* lexio.h (struct filespan_t): Mollify cppcheck with a const variable.
	* parse.y: Adjust a dbgmsg() call.
	* util.cc (cobol_filename): Likewise.

libgcobol/ChangeLog:

	* gmath.cc (scale_int256_by_digits): Switch to int256::rdigits.
	(add_int256_to_int256): Likewise.
	(squeeze_int256): Likewise.
	(get_int256_from_qualified_field): Likewise.
	(__gg__add_fixed_phase1): Likewise.
	(__gg__addf1_fixed_phase2): Likewise.
	(__gg__fixed_phase2_assign_to_c): Likewise.
	(__gg__addf3): Likewise.
	(__gg__subtractf1_fixed_phase2): Likewise.
	(__gg__subtractf2_fixed_phase1): Likewise.
	(__gg__subtractf3): Likewise.
	(multiply_int128_by_int128): Likewise.
	(__gg__multiplyf1_phase2): Likewise.
	(__gg__multiplyf2): Likewise.
	(divide_int128_by_int128): Likewise.
	(__gg__dividef1_phase2): Likewise.
	(__gg__dividef23): Likewise.
	(__gg__dividef45): Likewise.