scripts: update-oui: Add OUI list update script

While trying out netsniff-ng in my home network, I noticed that many OUIs were not resolved. While this is of course only a cosmetic problem, it would still be nice to have this information. Instead of manually patching the file, I created a script to fetch the necessary information from the IEEE website and write an oui.conf in the format netsniff-ng expects.

While at it, use the script to update src/conf/oui.conf

Commit message follows below:

Instead of having to maually update the oui.conf file used by
netsniff-ng to resolve OUIs to vendor names, provide a script which
fetches the necessary information from any file in the format as
provided by the IEEE at
http://standards.ieee.org/develop/regauth/oui/oui.txt

This is also the default URL the script will fetch the information from.

The script runs with Python 2.x and 3.x and only uses the respective
standard libraries.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>