New config variable: apache_extra_conf

Allows the user to specify bits to be placed in the Apache config file
directly, without requiring a separate include file.

Use case: for testing in Vagrant I need to specify some SSL certificate.
It's easier if I can just uncomment the example.
Make pov-update-server-page -v say what it's creating
Print names of files only if they were modified

Speaking of which, actually implement the "do not modify if contents do
not change" logic that was already documented in the docstring.  Oops.