Replace CONFIG_FILENAME with CONFIG_PATH

CONFIG_FILENAME works relative to the app's instance path. While (strictly
speaking) CONFIG_FILENAME is named correctly, it is not really obvious that
it should be set to an instance-relative filename instead of a path. The
current uwsgi.ini file illustrates this problem.

Not having a way to specify an absolute config file path is a problem for
the Debian package: The actual config file /etc/uffd/uffd.cfg must be
symlinked to /usr/share/uffd/instance/config.cfg to be found. Setting
CONFIG_PATH to "/etc/uffd/uffd.cfg" simplifies this.

Since this change is part of a new major release, we can drop
CONFIG_FILENAME in favour of CONFIG_PATH.