net: move IsReachable() code to netbase and encapsulate it

`vfLimited`, `IsReachable()`, `SetReachable()` need not be in the `net`
module. Move them to `netbase` because they will be needed in
`LookupSubNet()` to possibly flip the result to CJDNS (if that network
is reachable).

In the process, encapsulate them in a class.

`NET_UNROUTABLE` and `NET_INTERNAL` are no longer ignored when adding
or removing reachable networks. This was unnecessary.

src/netbase.h

@@ -19,6 +19,7 @@
 #include <stdint.h>
 #include <string>
 #include <type_traits>
+#include <unordered_set>
 #include <vector>
 
 extern int nConnectTimeout;
@@ -64,6 +65,61 @@ struct ProxyCredentials
     std::string password;
 };
 
+/**
+ * List of reachable networks. Everything is reachable by default.
+ */
+class ReachableNets {
+public:
+    void Add(Network net) EXCLUSIVE_LOCKS_REQUIRED(!m_mutex)
+    {
+        AssertLockNotHeld(m_mutex);
+        LOCK(m_mutex);
+        m_reachable.insert(net);
+    }
+
+    void Remove(Network net) EXCLUSIVE_LOCKS_REQUIRED(!m_mutex)
+    {
+        AssertLockNotHeld(m_mutex);
+        LOCK(m_mutex);
+        m_reachable.erase(net);
+    }
+
+    void RemoveAll() EXCLUSIVE_LOCKS_REQUIRED(!m_mutex)
+    {
+        AssertLockNotHeld(m_mutex);
+        LOCK(m_mutex);
+        m_reachable.clear();
+    }
+
+    [[nodiscard]] bool Contains(Network net) const EXCLUSIVE_LOCKS_REQUIRED(!m_mutex)
+    {
+        AssertLockNotHeld(m_mutex);
+        LOCK(m_mutex);
+        return m_reachable.count(net) > 0;
+    }
+
+    [[nodiscard]] bool Contains(const CNetAddr& addr) const EXCLUSIVE_LOCKS_REQUIRED(!m_mutex)
+    {
+        AssertLockNotHeld(m_mutex);
+        return Contains(addr.GetNetwork());
+    }
+
+private:
+    mutable Mutex m_mutex;
+
+    std::unordered_set<Network> m_reachable GUARDED_BY(m_mutex){
+        NET_UNROUTABLE,
+        NET_IPV4,
+        NET_IPV6,
+        NET_ONION,
+        NET_I2P,
+        NET_CJDNS,
+        NET_INTERNAL
+    };
+};
+
+extern ReachableNets g_reachable_nets;
+
 /**
  * Wrapper for getaddrinfo(3). Do not use directly: call Lookup/LookupHost/LookupNumeric/LookupSubNet.
  */