Kernel: Add a global routing table

Previously the system had no concept of assigning different routes for
different destination addresses as the default gateway IP address was
directly assigned to a network adapter. This default gateway was
statically assigned and any update  would remove the previously existing
route.

This patch is a beginning step towards implementing #180. It implements
a simple global routing table that is referenced during the routing
process. With this implementation it is now possible for a user or
service (i.e. DHCP) to dynamically add routes to the table.

The routing table will select the most specific route when possible. It
will select any direct match between the destination and routing entry
addresses. If the destination address overlaps between multiple entries,
the Kernel will use the longest prefix match, or the longest number of
matching bits between the destination address and the routing address.
In the event that there is no entries found for a specific destination
address, this implementation supports entries for a default route to be
set for any specified interface.

This is a small first step towards enhancing the system's routing
capabilities. Future enhancements would include referencing a
configuration file at boot to load pre-defined static routes.

Kernel/API/POSIX/net/route.h

@@ -14,6 +14,7 @@ extern "C" {
 #endif
 
 struct rtentry {
+    struct sockaddr rt_dst;     /* the target address */
     struct sockaddr rt_gateway; /* the gateway address */
     struct sockaddr rt_genmask; /* the target network mask */
     unsigned short int rt_flags;