This 35k message folder at work is getting burdensome. minc was

This 35k message folder at work is getting burdensome.  minc was
reading that entire folder everytime it stored a message there!  Twice
because mark(1) does it too.  The first is my fault, the second mh's.
I still haven't gone through mh to get rid of unnecessary readdirs, so
work around the problem here.

(get_highest_msgnum): Only read a directory once, saving the highest
message number and incrementing it on each call.

(mark): New function to add a message to a sequence.

(store_message): Use mark instead of running mark(1).