A series of commits spread these out throughout the tree, rather than using .PATH reachover or some other mechanism to achieve the needed effect. This was a fairly contentious move, and many have expressed (probably mostly privately) desire for the old structure to be largely restored.
This commit restores almost all of etc/ that got moved out. libexec/rc tentatively remains untouched. Any files moved back to the etc/ top-level continue to be installed from the Makefile that currently handles installation, as it's easier to make sure they're installed in the correct package this way. Files added to subdirectories of etc/ have had their installation moved to etc/<subdir/Makefile instead, as most of these tend to be logically grouped anyways.
One exception to the old structure is that I did *not* reinstate etc/etc.<arch> just for /etc/ttys. The various ttys.<arch> files instead got moved to etc/ttys.<arch> and sbin/init reaches into etc/ for those.