rc.d/random: bring back rc.conf defaults, minor fixes

rc.d/random: bring back rc.conf defaults, minor fixes

 * Saving the entropy across reboots is good practice to
   avoid having a lack of entropy after restarting the
   machine. For some reason, it has been disabled by default
   for ages now.

 * This commit brings back the defaults in rc.conf to enable
   saving entropy across reboots.

 * Minor fixes to rc.d/random, such as not assuming
   /var/db/entropy is a good place to put the entropy file,
   last but not least because:
    - that location is not checked on the start part of the script,
      so it won't ever be used.
    - /var/db/entropy is most of the time a directory, for the
      cron-based entropy collection.