Add a whitelist of sockets on fork.

Maintain a whitelist of AF_UNIX sockets that are permitted
to exist at the time of forking. If an open socket does not belong
to the whitelist (or is not AF_UNIX), the process will abort. If an
open socket is whitelisted, it will be redirected to /dev/null after
a sucessful fork. This allows us to unify our handling of the special
zygote sockets (/dev/socket/zygote[_secondary]) with the existing
whitelist of non socket file descriptors.

This change also removes non-fatal ALOGW messages since they have the
side effect of reopening the logging socket.

bug: 30963384
Change-Id: Ie04dac62d0e0f29354df9ac15af217ad652ffbbe