Several tools handle --debug-gc command-line option, by explicitly
forcing GC on every exit path. This is tedious and prone to forgetting
some of the exit paths.
Instead, add a generic handler for --debug-gc, which installs an at_exit
hook to do the GC consistency check, and which can be called right in
the command-line parser. Also adjust all users of --debug-gc to use
that handler.
Signed-off-by: Roman Kagan <rkagan@virtuozzo.com>