[Mimedefang] max slaves report

[John Nemeth]

     I have a suggestion.  Would it be possible to have the multiplexor
keep track of the maximum number of slaves that have been busy at the
same time and report this number with 'md-mx-ctrl rawstatus'.  This
would be useful since it would let me know if I was in danger of
running out of slaves.

     Another idea would be to keep track of how many times a given
number of slaves was busy.  This would be useful for creating
histograms and to see what the minimum number of slaves should be.