Make getrusage() output a little more readable

The getrusage() output that is shown by VACUUM VERBOSE and other places
has always been a little too terse and confusing for me.  I propose to
improve that a bit:

-CPU 0.01s/0.08u sec elapsed 0.18 sec.
+CPU: user: 0.08s, system: 0.01s, elapsed: 0.18s.

Patch attached.

I have also updated the ShowUsage() output used by log_parser_stats etc.
to match this change, but I don't claim that that output is particularly
readable either way.

While updating the VACUUM man page I noticed that the example output
doesn't match exactly anymore, but I didn't do anything about that in
this patch, just changed the lines that are affected by this patch.

-- 
Peter Eisentraut              http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services