raco pkg: configuration inherited: installation -> shared -> user
make packages single-collection by default

The `single-collection' "info.rkt" definition is no longer supported.
scribblings/main: fix installation start page to avoid user-specific packages
raco setup: fix handling of 'lib paths in "info-cache.rktd"
add `site' makefile target

Also, adjust stamp handling to ensure that it doesn't change
during the build process.
configure: add --enable-sdk6

To ensure that the Mac OS X 64-bit builds work on 10.6.
pkg: doc repairs
distro-build/assemble-site: fill in missing catalog pieces
Run more tests in Travis.