GN_D16, 16bit unsigned division

IN:
     HL = dividend
     DE = divisor

OUT, if call successful:
     Fc = 0
     HL = quotient
     DE = remainder

OUT, if call failed:
     Fc = 1
     A = error code:
          RC_FAIL ($16), division by zero attempted

Registers changed after return:
     ..BC..../IXIY same
     AF..DEHL/.... different