* ext/gdbm/gdbm.c (fgdbm_closed): new method GDBM#closed?
* ext/sdbm/init.c (fsdbm_closed): new method SDBM#closed?
* test/dbm/test_dbm.rb, test/gdbm/test_gdbm.rb, test/sdbm/test_sdbm.rb
(teardown): close all db objects before deleting data files.
* win32/win32.{ch} (unlink): hook runtime function to change
file attribute before unlinking.
merge from 1.8, see [ruby-dev:26360]
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@8650 b2dd03c8-39d4-4d8f-98ff-823fe69b080e