Ported 42075 from release23-maint branch.

SF bug #1403349 solution for email 3.0; some MUAs use the 'file' parameter
name in the Content-Distribution header, so Message.get_filename() should fall
back to using that.  Will port to the Python 2.5 trunk.

Also, bump the email package version to 3.0.1 for eventual release.  Of
course, add a test case too.

XXX Need to update the documentation.