AW: Cleaning up include file inconsistencies

[Dr. Matthias St. Pierre]

 A good idea just occurred to me. I will rework #9274 and create two
new pull requests from it:

- PR 1:  restructure the internal headers and fix the internal include guards.
- PR 2:  fix the include guards for the public header files

PR 1 could be backported to 1.1.1 which would be advantageous for cherry-picking.
That's important IMHO because 1.1.1 is an LTS release.

PR 2 would only go to master.

What do you think about it?

Matthias