examples/nettest: add socket close for udp test
examples/nettest: add socket close for udp test

This patch include add socket close for udp test. When 'UDP client'
terminated the message, 'UDP server' did not close the socket. When the
message for 'UDP client' is finished, it will be automatically modified
to terminate 'UDP server'. 'TCP server / TCP client' works normally.

Change-Id: Ia0649dd9ef29589f62dcc5c1e03174823fa285c8
Signed-off-by: Junhwan Park <7fee488b6f78fc50b291ffc7d10428788977e877@samsung.com>

JunhwanPark/TizenRT,JunhwanPark/TizenRT,JunhwanPark/TizenRT,JunhwanPark/TizenRT,JunhwanPark/TizenRT,JunhwanPark/TizenRT