facilitate -Dsolr.log.muteconsole opt-out

Having made a small log4j2.xml edit

 <Root level="info">
   <AppenderRef ref="RollingFile"/>
-  <AppenderRef ref="STDOUT"/>
+  <AppenderRef ref="STDOUT" level="error"/>
 </Root>

I was surprised to find the errors logged in the solr.log file but not in the solr-*-console.log file.

https://lucene.apache.org/solr/guide/7_5/configuring-logging.html#permanent-logging-settings very helpfully mentioned how the console logger is disabled when running in the background.

This ticket proposes to facilitate opting out of the muting via a SOLR_LOG_MUTECONSOLE_OPT option.