Use trim() to strip whitespace from the end of header fields

The main motivation was to ease adding attachments. You'll easy happen
to have trailing whitespace there, whereas you'll probably never want
to send files that have a name with whitespace at the end ... at least
you should never want to have or even send such files.
I don't know if it is a good idea to strip the trailing whitespace
in `anno -list' as well. I couldn't come up with an example when I
would want to keep it, hence I decided to remove it. This can be
changed back, if needed.