Another pass at cleaning up (some of) the manpages.

Another pass at cleaning up (some of) the manpages.

This continues work I started a couple of years ago, but then got
derailed on.  There are two main goals behind these updates:

1) replace some low-level troff constructs with more modern
   man(7) facilities, and

2) adopt a more consistent visual style to the layout and
   formatting of the manpages, particularly in reference to
   typeset pages.

This time around I have updated README.manpages to describe the new
guidelines.  They are mostly non-intrusive, and serve to clean up
a lot of visual clutter that has crept in over the years.