Remove header guards from what is now a source file

Pass chain as an arg, similar to what was done in D8369

Fixup help/error message for incorrect number of args

Pass chain as an arg instead of having separate functions

@bot build-ibd

Define a function that returns the vector of seeds in the autogenerated code

Move SeedSpec6 to more appropriate header file to fix circular dependency

Rebase

Fixup generate-seeds.py to match the rest of the changes

Please be sure to land with the rest of the stack.

I also want to point out that encouraging users to run pruned nodes isn't very important. The Core crowd will likely disagree, but this patch is clearly the right way to go to not introduce an avenue for accidental data loss. Users that are concerned about disk space should have their node configured accordingly.

See test failures

@bot gitian-linux gitian-osx gitian-win