I fixed the problems with people whose anydbm was using the dbm module at the

I fixed the problems with people whose anydbm was using the dbm module at the
backend. It turns out the dbm module modifies the file name to append ".db"
and my check to determine if we're opening an existing or new db just
tested os.path.exists() on the filename. Well, no longer! We now perform a
much better check _and_ cope with the anydbm implementation module changing
too!
I also fixed the backends __init__ so only ImportError is squashed.

git-svn-id: http://svn.roundup-tracker.org/svnroot/roundup/trunk@455 57a73879-2fb5-44c3-a270-3262357dd7e2