lualoader: Allow brand-*.lua for adding new brands

dteske@, I believe, had originally pointed out that lualoader failed to
allow logo-*.lua for new logos to be added. When correcting this mistake, I
failed to do the same for brands.

Correct the sub-mistake: creating new brands is almost identical to creating
new logos, except one must use drawer.addBrand and 'graphic' is the only
valid key for a branddef at the moment.

While here, I've added drawer.default_brand to be set to name of brand to
be used (e.g. 'fbsd', project default).

Eventually this whole goolash will be documented.

Reported by: kmoore, iXsystems