store ENV{key}="value" exported keys in the database

Just like the IMPORT values are stored in the udev database the
exported keys are stored in the udev database. The database is also
imported on a remove event before the rules are matched. So it
is possible to do:
  ACTION=="add", KERNEL=="sda", ENV{MY_DEVICE}="1234", RUN+="/bin/logger add $devpath"
  ACTION=="remove", ENV{MY_DEVICE}=="1234", RUN+="/bin/logger remove $devpath"

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