add flag for reading of precompiled rules

Rules can be precompiled and stored on disk for initramfs, to avoid
parsing the rules with every event again and again. Also the OWNER and
GROUP names are already resolved to numerical values in the compiled
rules. This flag is used for the upcoming move of the rules parsing
into udevd:
  If the real root is mounted udevd is started and parses the rules
  only once. The event processes will inherit the already parsed rules
  from the daemon, so we want to ignore any precompiled rules and
  use the real rules files and watch the filesystem for changes to
  reload the rules automatically.

Signed-off-by: Kay Sievers <kay.sievers@suse.de>