doom: support common auto-loaded files for all Doom flavors

[fabiangreffrath]

Fixes: #1182

[fabiangreffrath]

To provide some more context: I have a similar feature in Crispy that I'd currently like to get rid of, but it applies to all IWADs.

[fabiangreffrath]

I'll merge this after the weekend if nobody objects until then.

[fragglet]

Seems good. Only question: is "common" a good name? These only get loaded with Doom IWADs, not Heretic/Hexen/Strife - this is correct behavior but the name might suggest they would get loaded with all of them.

I can't think of a better name OTOH so I'm happy going with "common" but maybe you can think of something.

[fabiangreffrath]

We could add "doom" to the directory name and call it something like
doom-common
common-doom
doom-any
any-doom
doom-all
all-doom

or whatever you prefer. 😉

[fragglet]

Let's go with doom-all then.

[fabiangreffrath]

Done so, thanks!