contrib/openrc/init.d/package-cache: Add PC_OPTS

You can use this to tweak the logging:

  $ cat /etc/conf.d/package-cache
  PC_OPTS="-vvv"
main: Add the logger name and process ID to the syslog formatter

When logging to stderr, there's no need to differentiate the logging
process.  When everything's landing in the same system log, there is.