Add marking of DNS sockets for mark based routing

Adds an extra mark parameter to android_getaddrinfoforiface,
android_gethostbyaddrforiface and android_gethostbynameforiface that if set
will cause all packets sent by DNS requests to have that mark

Change-Id: I6f72390e4ce5bfc3cc73183f9b2fb3705a11685f
diff --git a/libc/private/resolv_private.h b/libc/private/resolv_private.h
index 0e799ca..61db8c8 100644
--- a/libc/private/resolv_private.h
+++ b/libc/private/resolv_private.h
@@ -175,6 +175,7 @@
 	res_send_qhook qhook;		/* query hook */
 	res_send_rhook rhook;		/* response hook */
 	int	res_h_errno;		/* last one set for this context */
+	int _mark;          /* If non-0 SET_MARK to _mark on all request sockets */
 	int	_vcsock;		/* PRIVATE: for res_send VC i/o */
 	u_int	_flags;			/* PRIVATE: see below */
 	u_int	_pad;			/* make _u 64 bit aligned */
@@ -496,6 +497,7 @@
 				    union res_sockaddr_union *, int);
 
 void res_setiface();
+void res_setmark();
 u_int  res_randomid(void);
 
 __END_DECLS