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/netbsd/resolv/res_init.c b/libc/netbsd/resolv/res_init.c
index ff65299..ceb412b 100644
--- a/libc/netbsd/resolv/res_init.c
+++ b/libc/netbsd/resolv/res_init.c
@@ -806,4 +806,11 @@
 		}
 	}
 }
+
+void res_setmark(res_state statp, int mark)
+{
+	if (statp != NULL) {
+		statp->_mark = mark;
+	}
+}
 #endif /* ANDROID_CHANGES */