Revised New Relic daemon logging approach to go to file instead of stderr.

Update the newrelic logging approach to include both the Agent and Daemon logs.

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. 

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.

@dzuelke 
I am not sure this one should populate from the provided NEW_RELIC_LOG.  As @dzuelke discovered, the apparent default for the New Relic addon is "stdout" for NEW_RELIC_LOG.  David found that this default is inconvenient in the context of the PHP Agent, as it junks up the output of the various tools used to configure and warm-up the instance (e.g., composer, php cli, etc.).

I am thinking that *if* NEW_RELIC_LOG stays at "stdout" (for legacy reasons) then perhaps this line should be changed to unconditionally set NEW_RELIC_LOG=stderr.  As of this moment, I believe the New Relic addon will set to NEW_RELIC_LOG=stdout, so this line will cause that to just be taken and not use the alternate default value:  stderr.