nanobsd: Fix typos

Fix a number of typos in the code or comments.
Files under dhcpd, embedded, pcengines, and rescue were intentionally
omitted. These directories will be reviewed at a later date.

PR: 269884
Reviewed by: imp, zlei
Approved by: emaste (mentor)
MFC after: 1 month
Differential Revision: https://reviews.freebsd.org/D47587