Add "close" socket option for immediately close socket after finish check captive portal.

Unexpected packets are observed after captive portal check. Due to the packets, modem can't fall-back to idle state (modem remained as active state).
It makes unnecessary power consumtion.
To prevent it, "close" option is added in sendHttpProbe()

Test: check socket close.
Change-Id: I5b6987071ad915081846504adbb209eb371fd1c3