Create a 7-line MSGINF file which includes the full path to the tag file,

in scenarios where the tagfile may be used (e.g. not editing extended file
descriptions). This should allow SlyEdit and other extenal message editors
to detect the support for the editor.tag file dynamically rather than by
using the Synchronet version number. If the 7th line is blank or non-existent,
then the external editor tag file is not supported.
Overhauled editor_inf() to use a file stream/fprintf.