Kernel: Add interface to read link speed and duplex for NetworkAdapter

Read the appropriate registers for RTL8139, RTL8168 and E1000.
For NE2000 just assume 10mbit full duplex as there is no indicator
for it in the pure NE2000 spec. Mock values for loopback.

Kernel/Net/NE2000NetworkAdapter.h

@@ -29,6 +29,12 @@ public:
         // just assume that it's up.
         return true;
     }
+    virtual i32 link_speed()
+    {
+        // Can only do 10mbit..
+        return 10;
+    }
+    virtual bool link_full_duplex() { return true; }
 
     virtual StringView purpose() const override { return class_name(); }