fileutils: move API to std::filesystem, cleanup

CMakeLists.txt

include/certstore.h

include/connectionmanager.h

include/diffie-hellman.h

include/fileutils.h

include/turn_cache.h

src/connectionmanager.cpp

src/fileutils.cpp

src/security/certstore.cpp

src/security/diffie-hellman.cpp

src/turn/turn_cache.cpp

tests/testFileutils.cpp