INT D4 - PC-MOS/386 v5.01 - GET CURRENT USERNAME AND SECURITY CLASS             
	AH = 1Eh
	BX = task ID (FFFFh for current task)
	ES:DI -> 4-byte buffer for username
Return: CF clear if successful
	    CL = security class
		20h (' ') none
		41h-5Ah ('A'-'Z') security level
	    ES:DI buffer filled
	CF set on error
	    AX = error code (see #01680 at INT 21/AH=59h/BX=0000h)
SeeAlso: AH=19h,AH=1Dh