EZP-23463: scripts/rhel/solr restart & stop does not work

https://jira.ez.no/browse/EZP-23463
( and originally also https://jira.ez.no/browse/EZP-23464 which reported the stop issue )

- /etc/init.d/solr restart now working ('2&>1' changed to '2>&1')
- replaced 'pidof java' by 'ps' and 'grep'
- replaced 'RETVAL=$?' after starting background job, as this would always be 0
- quoted variables, so they may contain whitespace