Make the update feature optional and disabled by default

[ 51%] Building C object src/CMakeFiles/lib_ettercap.dir/dissectors/ec_ssh.c.o

/home/travis/build/Ettercap/ettercap/src/dissectors/ec_ssh.c:560:52: error:

      variable 'm_h_n' is uninitialized when used here [-Werror,-Wuninitialized]

            temp_session_id=ssh_session_id(cookie, m_h_n, m_s_n);

                                                   ^~~~~

/home/travis/build/Ettercap/ettercap/src/dissectors/ec_ssh.c:146:29: note:

      initialize the variable 'm_h_n' to silence this warning

   BIGNUM *h_n, *s_n, *m_h_n, *m_s_n;

                            ^

                             = NULL

/home/travis/build/Ettercap/ettercap/src/dissectors/ec_ssh.c:560:59: error:

      variable 'm_s_n' is uninitialized when used here [-Werror,-Wuninitialized]

            temp_session_id=ssh_session_id(cookie, m_h_n, m_s_n);

                                                          ^~~~~

/home/travis/build/Ettercap/ettercap/src/dissectors/ec_ssh.c:146:37: note:

      initialize the variable 'm_s_n' to silence this warning

   BIGNUM *h_n, *s_n, *m_h_n, *m_s_n;

                                    ^

                                     = NULL

/home/travis/build/Ettercap/ettercap/src/dissectors/ec_ssh.c:552:54: error:

      variable 'h_n' is uninitialized when used here [-Werror,-Wuninitialized]

            temp_session_id = ssh_session_id(cookie, h_n, s_n);

                                                     ^~~

/home/travis/build/Ettercap/ettercap/src/dissectors/ec_ssh.c:146:15: note:

      initialize the variable 'h_n' to silence this warning

   BIGNUM *h_n, *s_n, *m_h_n, *m_s_n;

              ^

               = NULL

/home/travis/build/Ettercap/ettercap/src/dissectors/ec_ssh.c:552:59: error:

      variable 's_n' is uninitialized when used here [-Werror,-Wuninitialized]

            temp_session_id = ssh_session_id(cookie, h_n, s_n);

                                                          ^~~

/home/travis/build/Ettercap/ettercap/src/dissectors/ec_ssh.c:146:21: note:

      initialize the variable 's_n' to silence this warning

   BIGNUM *h_n, *s_n, *m_h_n, *m_s_n;

                    ^

                     = NULL

/home/travis/build/Ettercap/ettercap/src/dissectors/ec_ssh.c:398:64: error:

      variable 's_d' is uninitialized when used here [-Werror,-Wuninitialized]

               RSA_set0_key(session_data->serverkey, s_n, s_e, s_d);

                                                               ^~~

/home/travis/build/Ettercap/ettercap/src/dissectors/ec_ssh.c:148:21: note:

      initialize the variable 's_d' to silence this warning

   BIGNUM *h_d, *s_d, *m_h_d, *m_s_d;

                    ^

                     = NULL

/home/travis/build/Ettercap/ettercap/src/dissectors/ec_ssh.c:409:62: error:

      variable 'h_d' is uninitialized when used here [-Werror,-Wuninitialized]

               RSA_set0_key(session_data->hostkey, h_n, h_e, h_d);

                                                             ^~~

/home/travis/build/Ettercap/ettercap/src/dissectors/ec_ssh.c:148:15: note:

      initialize the variable 'h_d' to silence this warning

   BIGNUM *h_d, *s_d, *m_h_d, *m_s_d;

              ^

               = NULL

6 errors generated.

make[2]: *** [src/CMakeFiles/lib_ettercap.dir/build.make:1197: src/CMakeFiles/lib_ettercap.dir/dissectors/ec_ssh.c.o] Error 1