INT 21,12 - Search for Next Entry Using FCB

	AH = 12h
	DS:DX = pointer to unopened FCB returned from
		INT 21,11  or  INT 21,12

	on return:
	AL = 00 if file found
	   = FF if file not found

	- finds next matching file after calls to INT 21,11 and
	  INT 21,12
	- FCB should be the same across calls to INT 21,11 and 12
	- after successful call DTA holds an unopened FCB/XFCB for
	  the requested file.	Using any of the other FCB functions
	  destroys this DTA copy of the FCB/XFCB