Add some more functions to posix.cfg which allow to enable TestBufferOverrun::buffer_overrun_1_posix_functions
Fix some compiler warnings on MSVC
astyle formatting

[ci skip]
Fixed #5257 (Check memcpy size for string literals)
Fix MSVC compiler warnings
Avoid crash reported in #5943 (using the example from duplicate ticket #5971)
Replace a few size_t/unsigned int by std::size_t
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.