Network stack refactor

This introduces CNetAddr and CService, respectively wrapping an
(IPv6) IP address and an IP+port combination. This functionality used
to be part of CAddress, which also contains network flags and
connection attempt information. These extra fields are however not
always necessary.

These classes, along with logic for creating connections and doing
name lookups, are moved to netbase.{h,cpp}, which does not depend on
headers.h.

Furthermore, CNetAddr is mostly IPv6-ready, though IPv6
functionality is not yet enabled for the application itself.

src/uint256.h

@@ -623,7 +623,7 @@ inline const uint256 operator-(const uint256& a, const uint256& b)      { return
 
 
 
-
+#ifdef TEST_UINT256
 
 inline int Testuint256AdHoc(std::vector<std::string> vArg)
 {
@@ -756,3 +756,5 @@ inline int Testuint256AdHoc(std::vector<std::string> vArg)
 }
 
 #endif
+
+#endif