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/resolv/res_state_ext.h b/resolv/res_state_ext.h
new file mode 100644
index 0000000..cf906f0
--- /dev/null
+++ b/resolv/res_state_ext.h
@@ -0,0 +1,22 @@
+/*	$NetBSD: res_private.h,v 1.1.1.1 2004/05/20 17:18:54 christos Exp $	*/
+
+#ifndef NETD_RES_STATE_EXT_H
+#define NETD_RES_STATE_EXT_H
+
+#include "resolv_private.h"
+
+// TODO: consider inlining into res_state
+struct res_state_ext {
+    union res_sockaddr_union nsaddrs[MAXNS];
+    struct sort_list {
+        int af;
+        union {
+            struct in_addr ina;
+            struct in6_addr in6a;
+        } addr, mask;
+    } sort_list[MAXRESOLVSORT];
+    char nsuffix[64];
+    char nsuffix2[64];
+};
+
+#endif  // NETD_RES_STATE_EXT_H