Optimize assigning new PTR directory numbers where possible.
Optimize assigning new PTR directory numbers where possible.

In particular, only update the PTRs that we know we
have to.

Signed-off-by: Chris Lalancette <281cd07d7578d97c83271fbbf2faddb83ab3791c@gmail.com>

clalancette/pycdlib,clalancette/pyiso