Feature request: user data to store with context

[JetForMe]

It would be helpful to have be able to store some user data with the context, primarily for use in callback functions. E.g.:

MODBUS_API void modbus_set_user_data(modbus_t *ctx, void *user_data);
MODBUS_API void* modbus_get_user_data(modbus_t *ctx);

This allows a client of the API to store some context for use in a callback like modbus_rtu_set_custom_rts().

[stephane]

Related to #646