[exim] Wishlist: Added control over rejectlog

As I see it now, one can either have the rejectlog enabled (default) or
disabled (with "write_rejectlog false" )

It would be nice if one could leave the rejectlog on, but control what
is written to it -- either by listing the allowed headers to be logged,
listing the headers to be excluded, or choosing between a 'minimum set'
of headers and all headers.

Why? Because 50% of my exim_rejectlog are SpamAssassin headers.
Another 25% are misc. headers that don't affect any troubleshooting or
serve a valid purpose. If i could have a log file with 1/4 of what's
in there, it would be easier for me to view and manage.