Remove os.SEEK_SET lookups from all seek calls.
Remove os.SEEK_SET lookups from all seek calls.

This slows things down a lot since python always needs
to do the lookup in os for the number.  Since it is the
default, just assume it and get the speedup.

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

clalancette/pycdlib,clalancette/pyiso