Increase CORE_SIZE_LIMIT to 1GB

We can be more generous here as there still is a check against available
free space and when there would remain less than 50MB after the core is
saved, it is discarded:

if [ $((${freespace} - ${coresize_approx})) -lt 50000 ]; then
  omit_core=true
  logger "rich-core: dumping core might fill up disk - not dumping"
  ...