MDnsSdListener - pass SOCK_CLOEXEC to socketpair()

Working towards reducing the amount of cross-exec accidental
file descriptor leakage out of netd and into for example clatd.

Test: built/installed on crosshatch

Signed-off-by: Maciej Żenczykowski <maze@google.com>
Change-Id: I046bc2f4919d03fbc1d66289f0774d85603c3e81