Fix the UDP recvmmsg support

Previously, the netmgr/udp.c tried to detect the recvmmsg detection in
libuv with #ifdef UV_UDP_<foo> preprocessor macros.  However, because
the UV_UDP_<foo> are not preprocessor macros, but enum members, the
detection didn't work.  Because the detection didn't work, the code
didn't have access to the information when we received the final chunk
of the recvmmsg and tried to free the uvbuf every time.  Fortunately,
the isc__nm_free_uvbuf() had a kludge that detected attempt to free in
the middle of the receive buffer, so the code worked.

However, libuv 1.37.0 changed the way the recvmmsg was enabled from
implicit to explicit, and we checked for yet another enum member
presence with preprocessor macro, so in fact libuv recvmmsg support was
never enabled with libuv >= 1.37.0.

This commit changes to the preprocessor macros to autoconf checks for
declaration, so the detection now works again.  On top of that, it's now
possible to cleanup the alloc_cb and free_uvbuf functions because now,
the information whether we can or cannot free the buffer is available to
us.

lib/isc/netmgr/tcpdns.c

@@ -879,6 +879,15 @@ isc__nm_tcpdns_read_cb(uv_stream_t *stream, ssize_t nread,
 
 	isc__nm_process_sock_buffer(sock);
 free:
+	if (nread < 0) {
+		/*
+		 * The buffer may be a null buffer on error.
+		 */
+		if (buf->base == NULL && buf->len == 0) {
+			return;
+		}
+	}
+
 	isc__nm_free_uvbuf(sock, buf);
 }