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.cpp

@@ -32,6 +32,8 @@ bool fNameLookup = DEFAULT_NAME_LOOKUP;
 std::chrono::milliseconds g_socks5_recv_timeout = 20s;
 static std::atomic<bool> interruptSocks5Recv(false);
 
+ReachableNets g_reachable_nets;
+
 std::vector<CNetAddr> WrappedGetAddrInfo(const std::string& name, bool allow_lookup)
 {
     addrinfo ai_hint{};