bionic: Replace iface with opaque netid in resolver.

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
diff --git a/libc/dns/resolv/res_state.c b/libc/dns/resolv/res_state.c
index 087a5e6..01f68e9 100644
--- a/libc/dns/resolv/res_state.c
+++ b/libc/dns/resolv/res_state.c
@@ -54,6 +54,7 @@
 
 typedef struct {
     int                  _h_errno;
+    // TODO: Have one __res_state per network so we don't have to repopulate frequently.
     struct __res_state  _nres[1];
     unsigned             _serial;
     struct prop_info*   _pi;