[PATCH]: Add --password-file and --password-envvar

Attached is a log message and a patch which adds the new options
'--password-file' and '--password-envvar'. It also adds Julians warning
to the '--password' help text.

I haven't found out yet how the test suite checks the '--password'
option. So I tested it just by hand :)

Also I added a check that stdin is specified only once. This should
probably go into a seperate patch.

This is just the first iteration of the patch. Comments welcome.

- Martin