[exim-dev] wish - filter include capability

- make filter files incudable, such as:

#filter.main:
include /etc/exim4/filter/filter_start
include /etc/exim4/filter/filter_virus
include /etc/exim4/filter/filter_spam1
include /etc/exim4/filter/filter_spam2
include /etc/exim4/filter/filter_spam3
include /etc/exim4/filter/filter_etc...
include /etc/exim4/filter/filter_end

G.