lib/socket: don't fail if no more local ports can be assigned in nl_socket_alloc

lib/socket: don't fail if no more local ports can be assigned in nl_socket_alloc

By failing inside of nl_socket_alloc(), the user can not even work around
when running out of local ports. This patch changes that if there are no more
local ports, we set the port to UINT32_MAX. This is a consistent behavior
to calling nl_socket_set_local_port(sk, 0).

In general, since nl_socket_set_local_port() does not restict the generated
ports in any way we cannot assume to have a valid port. So the check in
the constructor was harmful and users who ever encountered it (because they
created 1024 libnl3 sockets) could not even work around it.

Acked-by: Thomas Graf <tgraf@suug.ch>
Signed-off-by: Thomas Haller <thaller@redhat.com>