Allow a "max message count" to be specified in AreaMgr "rescan" requests

- The -R option (in the subject), can now include a message count
  i.e. -R=<count>
  I don't think this follows any existing area manager example usage, but I
  figured as long as we support a -R option, -R=<count> makes sense.
- The %RESCAN function (in the body) can now include a message count
  i.e. %RESCAN [area-tag] [R=<count>]
  This mimics the Mystic area manager usage, though we don't yet support
  a "D=days" option.

This at least partially addresses issue #929, but I sure would like some
people to test it an let me know how well it works (!)