Modbus feature 639 v1.3

This pull request adds support for Modbus protocol.

2 new keywords : modbus.function and modbus.access.
This should return ALPROTO_UNKNOWN instead of failed. We may retry once we have enough data. In TCP we have to consider the case where we can get less bytes than we expect now, but the rest later.

If you return unknown here, the proto detect code will send the full data again to retry detection.