Improve parsing of IPv6 addresses

Implement the following rules:
* Interpret [X]:Y as host=X port=Y, if Y is an integer
* Interpret X:Y as host=X port=Y, if Y is an integer and X contains no colon
* Interpret X:Y as host=X:Y port=default otherwise

src/netbase.h

@@ -133,6 +133,7 @@ class CService : public CNetAddr
 };
 
 enum Network ParseNetwork(std::string net);
+void SplitHostPort(std::string in, int &portOut, std::string &hostOut);
 bool SetProxy(enum Network net, CService addrProxy, int nSocksVersion = 5);
 bool GetProxy(enum Network net, CService &addrProxy);
 bool IsProxy(const CNetAddr &addr);