INT 21 - LANtastic v3+ - GET USERNAME FROM ACCOUNT FILE
AX = 5FB2h
BX = username entry index (0 for first)
DS:SI -> 16-byte buffer for username
ES:DI -> ASCIZ server name in form "\\machine"
Return: CF clear if successful
CF set on error
AX = error code
BX = next queue entry index


215FB2 - LANtastic v3+ - GET USERNAME FROM ACCOUNT FILE