Move /etc/default/grub to /etc/grub as it's owned by the sysadmin.

Move /etc/grub.d to /etc/default/grub.d as it's owned by the system.

Support /etc/grub's GRUB_CMDLINE_SORTIX in 10_sortix.

Remove the old /etc/grub.d/10_sortix.cache with a compatibility hook as it
has moved to /etc/default/grub.d/10_sortix.cache.