Despite the name, most of resolv_private.h is actually in glibc; it's just
that there it's tucked away in libresolv whereas we have it in libc.
Change-Id: Ibcf6603f1141ffd2661cee0bffa83052d4f9e549
Replace iface cache key with netid.
Replace _mark with netid.
Mark sockets used to test IPv4/IPv6 support as well as sockets
used to determine source address for rfc6724 sort.
Remove pid/uid mappings (functionality moved to system/netd).
Do not create resolv_cache when it does not exist, but rather only
when DNS configuration is supplied.
Clean up unused functions.
Change-Id: I9ccfda2902cc0943e87c9bc346ad9a2578accdab
