grubcfg_proxy strips --class from submenu commands

[Bion Pohl]

Unlike the menuentry command, the --class options on submenu are not passed by grubcfg_proxy.

Reproduction:

1. Fix /etc/grub.d/proxifiedScripts/os-prober so that it writes a "--class os" parameter as part of a submenu command. ('os' being an arbitrary class. I made a 'submenu.png' icon in my theme and when I write '--class submenu' into my grub.cfg it works.)

2. Edit XX_os-prober_proxy to have this:

'/etc/grub.d/proxifiedScripts/os-prober' | /etc/grub.d/bin/grubcfg_proxy "+*
"

3. Run update-grub and look at grub.cfg (or use grub-emu) and see that the '--class' is gone and there is no icon.

4. Edit XX_os-prober_proxy to have this:

'/etc/grub.d/proxifiedScripts/os-prober'

4. Run update-grub again and look at grub.cfg (or use grub-emu) and see that the '--class' is there and the icon is shown.