Configure FTL's MACVENDORDB setting to "/macvendor.db" on startup

[PromoFaux]

What does this PR aim to accomplish?:

Discovered while troubleshooting #1299 (See comment)

We used to symlink this file, however I hadn't realised at the time it was configurable in FTL's config (RTFM, Adam). Setting the config option is performed during the ensure_basic_configuration step, and is ignored if a user has passed in their own FTLCONF_MACVENDORDB environment variable

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