This commit adds an as-of-yet untested implemented of UDP write buffering.

Squashed commit of the following:

    net/udp:  Address most of the issues with UDP write buffering.  There is a remaining issue with handling one network going down in a multi-network environment.  None of this has been test but it is certainly ready for test.  Hence, the feature is marked EXPERIMENTAL.
    net/udp:  Some baby steps toward a corrected write buffering design.
    net/udp:  Remove pesky write buffer macros.
    Eliminate trailing space at the end of lines.
    net/udp:  A little more UDP write buffering logic.  Still at least on big gaping hole in the design.
    net/udp:  Undefined CONFIG_NET_SENDTO_TIMEOUT.
    net/udp:  Crude, naive port of the TCP write buffering logic into UDP.  This commit is certainly non-functional and is simply a starting point for the implementatin of UDP write buffering.
    net/udp:  Rename udp/udp_psock_sendto.c udp/udp_psock_sendto_unbuffered.c.

include/nuttx/net/net.h

@@ -208,8 +208,8 @@ struct socket
 
   FAR const struct sock_intf_s *s_sockif;
 
-#ifdef CONFIG_NET_TCP_WRITE_BUFFERS
-  /* Callback instance for TCP send */
+#if defined(CONFIG_NET_TCP_WRITE_BUFFERS) || defined(CONFIG_NET_UDP_WRITE_BUFFERS)
+  /* Callback instance for TCP send() or UDP sendto() */
 
   FAR struct devif_callback_s *s_sndcb;
 #endif