access "native-pkgs" as a git submodule

This looks like a good use case for submodules, because the
native-library packages change infrequently, and no one cares
about the history of changes relative to the rest of the
project (except to be able to get a set of packages that is
consistent with the rest of the repository).

The change avoids the problem that `make' on Mac OS X would
try a `git update', which is no good if you happen to be
offline (and have your "buidl.native-pkgs" repo pull from the
obvious place).

Finally, it's easier for Windows users, since `git submodule init'
and `git submodule update' is easier to use and remember than
`git clone <some-repository-URL>'. The makefile more helpfully
complains if "native-pkgs" doesn't seem to have been initialized
as a submodule.
add `build-stamp' configuration and fix `repo-time-stamp/stamp'

The "repo-time-stamp" collection used to be omitted from a
release, but to keep things simpler, it's staying in the
"drracket" (and therefore "main-distribution") package. The
build process installs an empty `build-stamp' value (by default)
into a release build, or it installs a useful stamp for
a non-release build.
rename "core" makefile targets to "base"

Using "base" is more consistent.
adjust startup to avoid config file if collection links are disabled

The intent of this change is to avoid trying to read a configuration
file from a stand-alone executable.
misc doc updates

The updates are related to obscure changes for the package
system and installers.
Windows installer: offer to auto-launch DrRacket

The 'start-menu? aux key for launcher creation changed to 'start-menu,
with a real-number value indicating a request and precedence for
auto-launching (where a precedence is needed because only one application
can be auto-launched).
raco setup: print "summary of errors" before the summary of errors