coverage analysis improvements

Here is a patch series with some significant reworking and adjusting of
how the coverage analysis tools are run. The result should be that the
"make coverage" runs are faster and more robust, the results are more
accurate, and the code is simpler. Please see the individual patches
for details.

I have replaced "make coverage-html" with "make coverage" (but kept the
previous name for compatibility), since the old meaning of the latter
has gone away and was never very useful. Other than that, the usage of
everything stays the same.

I will add it to the commit fest. Testing with different versions of
tools would be especially valuable.

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