xlog: Optionally show more approval/rejection reasons

xlog: Optionally show more approval/rejection reasons

* Include the $cathow from the decision in the approval mail
* Append it to the event in the event.log
* Make report capable of filtering the reasons shown (by showing
  only a specified number of words, defaulting to 1 for approve
  and reject, which gives the previous behaviour)

Setting -rapprove=9 would show "everything" which includes the
identity of the moderator approving.  -rapprove=2 would show just auto
vs. manual

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>