etc: Set DISTBASE during distribute for sub-makes

[jrtc27]

Summary

This is normally done by bsd.subdir.mk in its default distribute target,
but since etc overrides it we don't get that behaviour. Currently it's
not needed, but share/man's makedb (recursed into by etc's afterinstall)
will need it in the following commit. Technically this is only needed
for install, but do this for distribution too for completeness and
consistency.