INT 21,22 - Random Write Using FCB

	AH = 22h
	DS:DX = far pointer to an opened FCB

	on return:
	AL = 00 if write successful
	   = 01 if diskette full or read only
	   = 02 if DTA is too small

	- write records to random location in file opened with FCB
	- FCB must be setup with drive id, filename, extension,
	  record position and record length before call
	- current record position field in FCB is not updated