net: samples: fix malformed payload when coap_client/server retransmit

Original coap_client implementation does not setup "appdatalen" of
net_pkt correctly and does not strip the IP + UDP headers when doing
the retransmit. This will result in malformed coap packet. Fix it by
adding a strip_headers() function to set appdatalen and get rid of
IP + UDP headers.

Signed-off-by: Robert Chou <robert.ch.chou@acer.com>