ipv6: introduce rx_address_v4_t and rx_port_t

Use typedefs for primative ipv4 addresses and port numbers.
Fix serveral bugs in the address type conversion functions.
Add more tests to the unit rx/addr-t unit test, and an example
function to show how to cope with ipv4-only legacy functions.

Change-Id: I03a2fe3c52c0a3d8b4d29fc09460b5d3ad9df917