INT 21 - Concurrent DOS v3.2+ - INSTALLATION CHECK

	AX = 4451h
Return: CF set if not Concurrent DOS
	    AX = error code (see #01680 at AH=59h/BX=0000h)
	CF clear if successful
	    AH = single-user/multiuser nature (see #01581)
		10h single-user
		    AL = operating system version ID (see #01580)
		14h multiuser
		    AL = operating system version ID (see #01579)
Notes:	as of Concurrent DOS/XM 5.0 (possibly earlier), the version is stored
	  in the environment variable VER and the OS type in variable OS
	use this function if you are looking for multiuser capabilities,
	  AX=4452h for single-user
	this function should never return the single-user values; however, an
	  application should be prepared to accept single-user values, i.e. it
	  should check the returned AH and AL separately
	CCI Multiuser DOS 7.22 returns AX=DX=1466h.
	DR DOS 3.31+ error returns with AX=0001h
SeeAlso: AX=4452h,AX=4459h