Warn about any reference to /lib/init in init.d scripts

/lib/init/rw has gone away, and the scripts in /lib/init/*.sh represent
internal implementation details of initscripts, *not* public API for
other packages to use.  Nonetheless, some packages incorrectly reference
those scripts, notably /lib/init/vars.sh.

Signed-off-by: Niels Thykier <niels@thykier.net>