debian: Fix confusion between dp0 and of0.

debian: Fix confusion between dp0 and of0.

Earlier, 'nl:0' was changed to read 'dp0' in the Debian init script, but
this didn't take into account that this would also change the name of the
"local port" for the datapath from 'of0' to 'dp0'.

The "cleanest" fix would probably be to change all the instances of of0
to dp0, but this would also require changing the names of files in the
file system (e.g. /etc/openflow-switch/of0-cert.pem), so it's easier to
just change the dp0 instances to of0.  Thanks to Reid for suggesting this
simpler fix.

Fixes bug #1056.