init: introduce I2P connectivity options

Introduce two new options to reach the I2P network:

* `-i2psam=<ip:port>` point to the I2P SAM proxy. If this is set then
  the I2P network is considered reachable and we can make outgoing
  connections to I2P peers via that proxy. We listen for and accept
  incoming connections from I2P peers if the below is set in addition to
  `-i2psam=<ip:port>`

* `-i2pacceptincoming` if this is set together with `-i2psam=<ip:port>`
  then we accept incoming I2P connections via the I2P SAM proxy.

src/net.h

@@ -14,6 +14,7 @@
 #include <compat.h>
 #include <crypto/siphash.h>
 #include <hash.h>
+#include <i2p.h>
 #include <net_permissions.h>
 #include <netaddress.h>
 #include <optional.h>
@@ -831,6 +832,7 @@ public:
         std::vector<std::string> m_specified_outgoing;
         std::vector<std::string> m_added_nodes;
         std::vector<bool> m_asmap;
+        bool m_i2p_accept_incoming;
     };
 
     void Init(const Options& connOptions) {
@@ -1221,8 +1223,20 @@ private:
     Mutex mutexMsgProc;
     std::atomic<bool> flagInterruptMsgProc{false};
 
+    /**
+     * This is signaled when network activity should cease.
+     * A pointer to it is saved in `m_i2p_sam_session`, so make sure that
+     * the lifetime of `interruptNet` is not shorter than
+     * the lifetime of `m_i2p_sam_session`.
+     */
     CThreadInterrupt interruptNet;
 
+    /**
+     * I2P SAM session.
+     * Used to accept incoming and make outgoing I2P connections.
+     */
+    std::unique_ptr<i2p::sam::Session> m_i2p_sam_session;
+
     std::thread threadDNSAddressSeed;
     std::thread threadSocketHandler;
     std::thread threadOpenAddedConnections;