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).
