Redirect stderr to /dev/null when executing a command

fix #2545
Use xdg-open by default in the /link plugin

fix #2550
Fix a traceback when we trigger the reload using a signal
Add the pipe_cmd plugin

This plugins lets the user manipulate a poezio instance by writing commands
into a pipe, read by poezio which will execute them.
Make the thread from the cmd_plugin be a daemon

Without it, poezio never exits when the plugin is loaded

Thanks to xunien

fix #2542
Fix #2554 (traceback on last activity in the roster)