Fix excessive white space in manpages.

Fix excessive white space in manpages.

In nroff manpages, a blank line adds vertical white space.  When this is
followed by another command that also starts a new paragraph, the result
is a vertical skip twice as big as the normal inter-paragraph gap.  The
solution is to use a line that contains just "." for white space within
the manpage, instead of a blank line.  The resulting manpages look better.