Revised logging approach; and augmented system INI settings

The agent writes to stderr.

The daemon writes to a well-known log file and then a .profile.d script is created
which tails the output of this file to stderr.

This approach enables 'heroku logs' to get all the New Relic logging for *both* web
and worker instances.  Reverted the bin/* scripts back to their prior state (i.e., no
knowledge of the newrelic logs).  With this new approach, that is no longer needed.

Finally, introduced more system INI variables and documentation in the
ext-newrelic.ini and corresponding environment variables (with defaults) in the
newrelic.sh .profile.d script.