INT 2F - PTS-DOS 6.51, S/DOS 1.0+ - GET FIRST UMB

	AX = 1261h
Return: AX = address of first UMB or 1 if invalid function.
Note:	This function is known to be supported by Paragon Technology Systems
	  PTS-DOS 6.51 and S/DOS 1.0, and probably was also supported
	  in earlier releases.