SerialUSB changes

0.10    CP2102N device added.
        Fix OS_Args reading FTDI device baud rate incorrectly.


0.10a   CDC device added.
        DTR dropped when closing.
        fixed 'Bad Request' / stalling problem with reading string
        descriptors.
        
0.11    Close open SerialUSB files using fileswitch handle before
        deregistering device - fixes undeletable file handle when
        disconnected or module is killed.
        
0.11a   Ioctl changed to return new value instead of previous value.
        CH340 handshake crash fix.

0.11b   IOCTL changed to return new value instead of previous as per spec.

0.11c   Rx stream woken up when rx stream created instead of after first byte is fetched.

0.12    Don't change dtr/rts lines at initialisation or finalisation.

0.13    bug fixes.

0.14    sync_data_toggle removed - caused problems with pi pico.
        CDC works with pi pico (picomite).

0.15    Improved ftdi receive performance.
        Distribution tidied up.

0.16    Make CDC work with all pico family firmware.
        CDC handles multiple devices on 1 device.
        Device removed error added.
        Removed setting of DTR for pico.
        SerialUSB_DevicesChanged swi added to detect whether
                a SerialUSB device is added or removed.

0.17    Fix detection of OUT endpoint in CDC device - fixes zeropage error.