MEDIUM: stream_sock: add a get_src and get_dst callback and remove SN_FRT_ADDR_SET

MEDIUM: stream_sock: add a get_src and get_dst callback and remove SN_FRT_ADDR_SET

These callbacks are used to retrieve the source and destination address
of a socket. The address flags are not hold on the stream interface and
not on the session anymore. The addresses are collected when needed.

This still needs to be improved to store the IP and port separately so
that it is not needed to perform a getsockname() when only the IP address
is desired for outgoing traffic.