Move resolver public headers to separate include path

Additionally rename functions and structs to eliminate those ugly
underscores from the public API (with the notable exception of
__res_params, which will come in its own separate change to keep this
one from growing too large).

Test: build, flash, then 'atest netd_integration_test'
Change-Id: I7e9b0ee6cdbec889e9c35b5a17b5daa65533686e
diff --git a/res_init.cpp b/res_init.cpp
index d222ac2..05404cb 100644
--- a/res_init.cpp
+++ b/res_init.cpp
@@ -88,12 +88,10 @@
 #include <string.h>
 #include <unistd.h>
 
-/* ensure that sockaddr_in6 and IN6ADDR_ANY_INIT are declared / defined */
-#include "resolv_netid.h"
+#include "netd_resolv/resolv.h"
+#include "res_state_ext.h"
 #include "resolv_private.h"
 
-#include "res_private.h"
-
 /* Options.  Should all be left alone. */
 #ifndef DEBUG
 #define DEBUG
@@ -163,7 +161,7 @@
     statp->qhook = NULL;
     statp->rhook = NULL;
     statp->_u._ext.nscount = 0;
-    statp->_u._ext.ext = (struct __res_state_ext*) malloc(sizeof(*statp->_u._ext.ext));
+    statp->_u._ext.ext = (res_state_ext*) malloc(sizeof(*statp->_u._ext.ext));
     if (statp->_u._ext.ext != NULL) {
         memset(statp->_u._ext.ext, 0, sizeof(*statp->_u._ext.ext));
         statp->_u._ext.ext->nsaddrs[0].sin = statp->nsaddr;
@@ -214,7 +212,7 @@
 static void res_setoptions(res_state statp, const char* options, const char* source) {
     const char* cp = options;
     int i;
-    struct __res_state_ext* ext = statp->_u._ext.ext;
+    res_state_ext* ext = statp->_u._ext.ext;
 
 #ifdef DEBUG
     if (statp->options & RES_DEBUG)