Fix constructing of setdscp opcode with tablearg keyword.

setdscp's argument can have zero value that conflicts with IP_FW_TARG
value. Always set high-order bit if parser doesn't find tablearg keyword.

Fix formatting of setfib opcode.

Zero fib is correct value and it conflicts with IP_FW_TARG.
Use bprint_uint_arg() only when opcode contains IP_FW_TARG,
otherwise just print numeric value with cleared high-order bit.