net: move MaybeFlipIPv6toCJDNS() from net to netbase

It need not be in the `net` module and we need to call it from
`LookupSubNet()`, thus move it to `netbase`.

src/netbase.h

@@ -307,4 +307,13 @@ bool Socks5(const std::string& strDest, uint16_t port, const ProxyCredentials* a
  */
 bool IsBadPort(uint16_t port);
 
+/**
+ * If an IPv6 address belongs to the address range used by the CJDNS network and
+ * the CJDNS network is reachable (-cjdnsreachable config is set), then change
+ * the type from NET_IPV6 to NET_CJDNS.
+ * @param[in] service Address to potentially convert.
+ * @return a copy of `service` either unmodified or changed to CJDNS.
+ */
+CService MaybeFlipIPv6toCJDNS(const CService& service);
+
 #endif // BITCOIN_NETBASE_H