Downstream might want to choose a different default value for
main.rc-manager setting (and it can does so, by compiling with
explicit resolvconf or netconfig support).
Make the default configurable at build-time and also embed it into
the manual page of "NetworkManager.conf".
https://bugzilla.redhat.com/show_bug.cgi?id=1337222
This way it's consistently used across all manual page without a need
for XSL templating.
Also, the entities file could in future possibly be used to template the
build-time configurables such as filesystem paths or bug tracker URL.