尝试在LwIP中实现静态路由表

在lwIP中实现路由表或转发表，让SNMP读取

IP中的路由表或转发表MIB经历了一些变化，如下：

最早的MIB是RFC1156，1990年，叫ipRoutingTable，

后来的MIB II是RFC1213，1991年，在ipRoutingTable表项中增加了掩码。

4.1.1.  RFC 1213，1991年，最初提出ipRouteTable

   The ipRouteTable object was originally defined in RFC 1213 [RFC1213].
   It was updated by ipForwardTable in RFC 1354 [RFC1354].

4.1.2.  RFC 1354，（IP Forwarding Table MIB）1992年，由ipRouteTable修改为ipForwardTable，并增加ipForwardNumber（转发表条数）

   The ipForwardTable object replaced the ipRouteTable object from RFC
   1213.  It was in turn obsoleted by the ipCidrRouteTable defined in
   RFC 2096 [RFC2096].

   In addition, RFC 1354 introduced ipForwardNumber.  This object
   reflects the number of entries found in ipForwardTable.  It was
   obsoleted by ipCidrRouteNumber, defined in RFC 2096.

4.1.3.  RFC 2096

   In RFC 2096, the ipCidrRouteTable and ipCidrRouteNumber were
   introduced.  The ipCidrRouteTable object supports multipath IP routes
   having the same network number but differing network masks.  The
   number of entries in that table is reflected in ipCidrRouteNumber.
   These objects are deprecated by the definitions contained in this MIB
   definition.

4.1.4.  RFC 2011 and 2465

   RFC 2011 [RFC2011] contains the ipRoutingDiscards object, which
   counts the number of valid routes that have been removed from the
   ipCidrRouteTable object.  The corresponding ipv6DiscardedRoutes
   object is defined in RFC 2465 [RFC2465].  These objects are
   deprecated in favor of the version-independent object
   inetCidrRouteDiscards defined in this MIB.