[Exim] system-filter update - please test

I have (finally) slammed out an updated system-filter.

I'd apprecaite it if people could give this the once over before I more
widely announce it.

Changes:-
    - added .lnk extension (much applause)
    - changed \w in filenames to . (locale problems)
    - changed build method - now the filter is built from several
      small library files
    - added GPL notices
    - changed some comments etc
    - updated sample regexp at end

    Nigel.