* Resynchronise with Debian. Remaining changes: - Show the grub menu and raise the menu timeout if other operating systems are installed. - Ask grub-installer/only_debian at medium priority. - Remove splash boot parameter unless debian-installer/framebuffer=true. - If / or /boot are on a removable device, install GRUB there by default. - Only mount /target/proc if it isn't already mounted. - Add user parameters to defoptions rather than kopt, so that they aren't used in the recovery mode option (closes: Malone #31905). - Support setting OVERRIDE_UNSUPPORTED_OS in the environment to force grub-installer to use its default MBR selection method despite there being unsupported operating systems on the disk.