drivers: nsos: support for AF_UNIX

Handle AF_UNIX family sockets for NSOS offloaded driver
Note that the size of struct sockaddr_un is done conditionnaly based on
CONFIG_NET_NATIVE_OFFLOADED_SOCKETS
Also, NET_SOCKADDR_PTR_MAX_SIZE needs to be updated only if
CONFIG_NET_SOCKETS_PACKET is not set. Otherwise, for a AF_PACKET socket,
a struct net_context variable can be corrupted, as local would have be on
16 bytes instead of 20 bytes.

Signed-off-by: Noemie Gillet <ngillet@sequans.com>