GN_Ghn, Convert hexadecimal string to 32 bits integer

IN :  HL = local source pointer
      DE = 2, return integer in DEBC
      DE > 255, return integer at (DE) and (DE)=LSB, (DE+3)=MSB

OUT:  Fc = 0, success 
      Fc = 1, A = RC_Bdn (bad hexadecimal string)
              A = RC_Fail (bad destination)
              A = RC_Hnd (bad handle for stream IX or filter IX)

Registers changed after return:
      A.BCDEHL/IXIY same
      .F....../.... different