since varnish-cache ecef48518f3b3f4bbf28256e090bdbb5cd2b163c backends
can be NULL (as defined with backend <name> None)

Fixes #1

configure checks if you have lcov & genhtml; these can be specified
with --with-lcov and/or --with-genhtml.

If they are available, then make coverage does the following:

- make clean, then make check with CC=gcc and CFLAGS set so that
  inputs for gcov/lcov are generated.

- lcov creates the src/coverage subdir and generates a targetfile
  there.

- genhtml generates HTML reports in src/coverage.

See also https://github.com/varnishcache/varnish-cache/issues/2967
for a general discussion regarding an inconsistency in the WS_Reserve()
interface

Because the non-manpage version currently produces a broken table
of contents.

Because the non-manpage version currently produces a broken table
of contents.

Even if it is pointless. All stats values are 0 in that case.