jp (shell): use of subprocess instead of running commands in the same process: when a command is calling host.quit() in a D-Bus callback, GLib mainloop is exiting and quit the whole process without (apparently?) any way to block it. Couldn't find a good way to avoid that, so subprocess are now used as a workaround, instead of launching commands in the same process. When we'll get rid of python-dbus, there may be an other way. Execuring external commands is now possible through the shell/! command.