o Added dev_open and dev_close functions

o  Added dev_open and dev_close functions

o  Changed disk-rep to use these

o  if NDEBUG is not defined the dev_cache will check for open devices on
   teardown.

I was hoping this would speed things up.  But I'm still getting:

reti:/home/joe/sistina/LVM2/tools# time ./lvm vgchange -a n
  Volume group vg0 successfully changed

real    0m5.751s
user    0m0.060s
sys     0m0.070s

even though I have only 1 device with the vg on it passing the filters.