apache: when sourcing env-enabled/*, redirect stdout to stderr

To determine its PID file, /etc/init.d/apache2 does the following:

  PIDFILE=$(. $APACHE_ENVVARS && echo $APACHE_PID_FILE)

This means that if the envvars script or any of the scripts it sources echoes
anything to stdout, Apache gets confused about its PID file.
We can protect against this by redirecting the output of env-enabled scripts to
stderr.

Change-Id: Ia0177ebf481a6db82c14f26bdd96cc0914d62ef0