ValueFlow: analysis in condition.
avr.cfg: Added support for some stdio.h macros.
htdocs: change link to trac
htdocs: removed files, they have been moved to the repo cppcheck-htdocs
Clean up .gitignore and remove the currently unneeded .gitattributes file.

[ci skip]
Update .gitignore.

Remove accidentally added file.

[ci skip]
Refactoring: Removed CheckNullPointer::nullPointerByCheckAndDeRef and implemented needed analysis in ValueFlow instead.
travis: replace kernel check with clang check.

Check clang instead of kernel, which is a lot faster.
Make clang check failure fatal to the build.
travis: add -DCHECK_INTERNAL to the VERIFY=1 job to automatically fetch issues like #5869 via travis, check clang with --exception-handling
travis: make clang check sensitive to internal cppcheck errors.