It handles exit() properly, calling the handler also when the plugin returns
normally (also it makes exit() more standard compliant while at it).
It also holds PLUGIN_HEADER, so that it doesn't need to be in each plugin anymore.
To work better together with callbacks passed to rb->default_event_handler_ex introduce exit_on_usb() which will call the exit handler before showing the usb screen and exit() after it.
In most cases it was passed a callback which was manually called at all other return points. This can now be done via atexit().
In future plugin_crt0.c could also handle clearing bss, initializing iram and more.
git-svn-id: svn://svn.rockbox.org/rockbox/trunk@27862 a1c6a512-1295-4272-9138-f99709370657
