added module comment about how overlay works
output to stdout
all new way of doing overlay

use sort() to always process most important entry first. "Duplicates"
are found using a set(). Postscripts are "confined" to the current
directory (before, they would be global)
visit(directory) and _run_post(object)
delete vnode uses destination stat

and single may search the delete tree too
and a supplied path must be a full path (or a terse path)
added TODO for terse paths
run post script on the parent dir if it had been updated
run .post script after --single
--single runs .post script on parent dir if it has one

It works because _walk_subtree() passes on the parent dir's .post
script to the subdir
run .post script on changed dir when files were deleted
--single works on delete tree, and may run delete .post scripts
return correct status
find_terse() works like before
print DRY_RUN message only once
fix rebase conflict
better description
overlay now works directly with SyncObjects

no more intermediate OverlayObjects
Merge branch 'overlay' into dev60
All new overlay code: This was needed because the multiple overlaydir
functionality in synctool 5 is an entirely different thing than the
grouped overlay dirs in synctool 6.