Add possibility to configure endpoints for all USB functions

Add function to set USB endpoints to use

Added
 void sanei_usb_set_endpoint (SANE_Int dn, SANE_Int ep_type, SANE_Int ep);
    
This function allows the user to set the endpoint to use on the given device for the given endpoint type (e.g. bulk in). Some scanners have multiple bulk in/out endpoints but use only one for communication (not the one detected during auto-detection), so one call to the new function will change the used endpoint.
Some scanners use two different endpoint for different  purposes, so in that case each read/write call needs to be preceeded by a call to that function to set the proper endpoint.
    
ep_type is a bitmask and is usually of the form
        (USB_DIR_IN|USB_ENDPOINT_TYPE_BULK)