INT 21 - LANtastic - LOGIN TO SERVER
        AX = 5F81h
        ES:DI -> ASCIZ login path followed immediately by ASCIZ password
        BL = adapter number
            FFh try all valid adapters
            00h-07h try only specified adapter
Return: CF clear if successful
        CF set on error
            AX = error code
Notes:  login path is of form "\\machine\username"
        if no password is used, the string at ES:DI must be terminated with
          three NULs for compatibility with LANtastic v3.0.
SeeAlso: AX=5F84h