sd-*.h: clean up exported (or to-be-exported) header files

sd-*.h: clean up exported (or to-be-exported) header files

Exported header files should not include internal headers. Fix that.

Exported header files should not use the bool type. So far we opted to
stick to C89 for exported headers, and hence use "int" for bools in
them. Continue to do so.

Exported header files should have #include lines for everything they use
including inttypes.h and sys/types.h, so that they may be included in
any order.

Exported header files should have C++ guards, hence add them.

Exported header files should not use gcc extensions like #pragma once,
get rid of it.