INT 21 - DOS 2+ - FREE MEMORY

	AH = 49h
	ES = segment of block to free
Return: CF clear if successful
	CF set on error
	    AX = error code (07h,09h) (see #01680 at AH=59h/BX=0000h)
Notes:	apparently never returns an error 07h, despite official docs; DOS 2.1+
	  code contains only an error 09h exit
	DOS 2.1-6.0 does not coalesce adjacent free blocks when a block is
	  freed, only when a block is allocated or resized
	the code for this function is identical in DOS 2.1-6.0 except for
	  calls to start/end a critical section in DOS 3.0+
SeeAlso: AH=48h,AH=4Ah