PR bin/60706 : sh(1) man page wording improvements.
Better describe how var-assign prefix words are processed.
A few other random cleanups in the nearby area.
No Dd update, these are not significant enough changes to suggest
to anyone that re-reading the man page would be worth the effort.
No pullups planned (same reason).
Allow the user to decline roll forward by passing option "-o nolog" to
mount_lfs. Internally this is implemented by interpreting the MNT_LOG
flag to mean "do roll forward". Option "-o log" defaults on in mount_lfs(8).
Instead of writing partial-segments synchronously when sync is requested,
write them async, but wait for vp->v_numoutput to drop to zero if the
calling semantics require it. That way, even if we might have to wait,
nobody else is required to wait with us.