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