net: app: client: handle client_addr port in net_app_init_client()

Currently, the client_addr parameter is only used to check the
sa_family at various points during the init process.  Both the
IP address and the port are ignored.  Let's set the local client's
port based on the port value of the passed in client_addr if one
is supplied.

Signed-off-by: Michael Scott <michael@opensourcefoundries.com>