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.
roff(7) dictates that "Each sentence should terminate at the end of an
input line." Instead of doing this, Sortix manpages (incorrectly) used
double-spaces to separate sentences.
Additionally, fix a few small typos.
The chain-merge target now invokes the /sysmerge init(8) with the merge
target, which now mounts the mountpoints. This change allows having a /boot
partition for the purpose of sysupgrade(8).