i2ptunnel: Enhance registration authentication page

- Enable form
- Switch to multipart form for getting old private key file
- Add advanced authentications
PrivateKeyFile: Add InputStream constructor for i2ptunnel

core/java/src/net/i2p/data/PrivateKeyFile.java

@@ -273,6 +273,18 @@ public class PrivateKeyFile {
         this.signingPrivKey = spk;
     }
     
+    /**
+     *  Can't be used for writing
+     *  @since 0.9.26
+     */
+    public PrivateKeyFile(InputStream in) throws I2PSessionException {
+        this("/dev/null");
+        I2PSession s = this.client.createSession(in, new Properties());
+        this.dest = s.getMyDestination();
+        this.privKey = s.getDecryptionKey();
+        this.signingPrivKey = s.getPrivateKey();
+    }
+    
     /**
      *  Create with the default signature type if nonexistent.
      *