Add support for user defined RTS toggle function

This patch allows users to register a custom callback function to set the RTS pin. This is handy on embedded systems where hardware RTS is not available, allowing the use of GPIO or raw register access instead.

By default it is mapped to call _modbus_rtu_ioctl_rts so it doesn't change the current functionality.

This is more or less a cleaned up version of remakeelectric@306e7ab for which I couldn't find a pull request. This relates to #156 and #23.