Make sure doasedit can handle filenames starting with a dash

This is a minor update which just affects the doasedit script, a utility to help the user cleanly edit files as another user.

In the past, doasedit would fail if the filename we were trying to edit started with a dash character (such as -bar.txt or -config.txt). It would also fail ungracefully if the user did not have permission to create a temporary file.

Now doasedit will exit gracefully with a useful error message if we are unable to create temporary files. It will also handle filenames with a leading dash.