Fix gazebo_ros scripts signal handling and do not load the gazebo_ros_api_plugin into the gzclient

[meyerj]

• Use exec where appropriate, such that signals send by roslaunch are received directly by the gzserver or gzclient processes and do not have to be forwarded by the shell explicitly.

• Apply the $gzserver_pid variable saved earlier instead of $! in the all-in-one gazeboscript to kill the server process once the client terminates. The old version should also have been fine, because$!is the pid of the last _background_ process. Before, variablegzserver_pid` was unused.

• With the gzclient script the gazebo_ros_api_plugin was also loaded into the client process. That plugin creates the /gazebo ROS node, and hence two identical nodes are spawned. Launching the client like that broke the ROS API of Gazebo for us, including /clock publishing, at least once the GUI would be closed.

  The gzclient script is also what is executed by the launch files in package gazebo_ros if argument gui:=true (the default). In this case the node names do not clash because roslaunch overwrites the default node name with /gazebo_gui, but still it is probably unintended to load the plugin into the GUI at all, I assume.

This patch eventually fixes #751?