DNS proxy thread in netd.

New thread in the netd process which listens on the
/dev/socket/dnsproxyd socket (group owned by 'inet', so only useable
by apps with the INTERNET permission), and does getaddrinfo requests
for other processes, caching in one place, rather than per-app.

Still remaining: proper caching based on DNS-requested lengths, upping
the cache size, getnameinfo, stats, flush, etc, etc.

Change-Id: I1d65af7d87876e508c718656bd81217cd961e20a