Determine IPv4 vs IPv6 processing in the UDP entity

[cliffg-softwarelibre]

There is a "TODO" in the net_ip/detail/udp_entity_io.hpp file that needs to be analyzed and implemented. The current code only supports IPv4 UDP datagrams, and this needs to be generalized for both IPv4 and IPv6.

[cliffg-softwarelibre]

The code has been changed so that the UDP socket open is not hard-coded for IPv4. However, there are still unknown (i.e. untested) use cases:

• When a local port is specified (for read processing), the format or construction of the endpoint determines whether it is IPv4 or IPv6 (if string used for endpoint, the string encoding determines protocol; if passed in as asio endpoint, the construction of the endpoint determines whether it is v4 or v6)
• When a "write-only" net-entity is created, there is not a local endpoint for binding; the open of the socket uses the default construction endpoint (which is likely v4)
• What happens when an application wants to send to multiple UDP destinations, first a v4 endpoint, then a v6 endpoint? If the code does not support this, we need to document