Modernize: Use #pragma once in all header files

#pragma once is a widely supported compiler pragma, even though it is not part of the C++ standard. Many of the issues keeping #pragma once from being standardized (distributed filesystems, build farms, hard links, etc.) do not apply to CMake - it is easy to build CMake on a single machine. CMake also does not install any header files which can be consumed by other projects (though cmCPluginAPI.h has been deliberately omitted from this conversion in case anyone is still using it.) Finally, #pragma once has been required to build CMake since at least August 2017 (7f29bbe6 enabled server mode unconditionally, which had been using #pragma once since September 2016 (b13d3e0d)). The fact that we now require C++11 filters out old compilers, and it is unlikely that there is a compiler which supports C++11 but does not support #pragma once.