Improve pid directory handling
Improve pid directory handling

fix security issue with mode 777 for pid directory
take default user for pid directory from config file
make pid handling configurable via /etc/default/newrelic_plugin_agent

ModusCreateOrg/newrelic-plugin-agent,alonisser/newrelic-plugin-agent,notnmeyer/newrelic-plugin-agent,rounds/newrelic-plugin-agent,rounds/newrelic-plugin-agent,NewRelic-Python-Plugins/newrelic-python-agent,whiteear/newrelic-plugin-agent,MeetMe/newrelic-plugin-agent,whiteear/newrelic-plugin-agent,notnmeyer/newrelic-plugin-agent,alonisser/newrelic-plugin-agent,ModusCreateOrg/newrelic-plugin-agent,ModusCreateOrg/newrelic-plugin-agent,NewRelic-Python-Plugins/newrelic-python-agent,rounds/newrelic-plugin-agent,alonisser/newrelic-plugin-agent,notnmeyer/newrelic-plugin-agent,NewRelic-Python-Plugins/newrelic-python-agent,whiteear/newrelic-plugin-agent,MeetMe/newrelic-plugin-agent,MeetMe/newrelic-plugin-agent