logd_write: set SOCK_CLOEXEC on socket

Socket file descriptors remain open across exec unless
SOCK_CLOEXEC is set. Enable this option, to avoid leaking
file descriptors.

See https://android-review.googlesource.com/53736 for a similar
problem with the old logging code.

Change-Id: I9e045d2291ae6680044ab86604f3ff2c55b5eaed