Fix unbound_monitor.sh (Ticket #2817)

. Use a PID file to keep track if it's running or not
. Move it to /usr/local/bin, it doesn't need to be started by
  rc.start_packages since it's already called by unbound.inc
. Define PATH and remove complete path for all binaries
. Remove unbound initscript symlink created when it's installed by a PBI
  package
. Bump unbound to 1.4.20_5