Add legacy include guards to public header files

Add legacy include guards to public header files

For every public header file, the old include guard definition needs
to be added in addition to the new one

include/openssl/file.h:

    #ifndef OPENSSL_FILE_H
    # define OPENSSL_FILE_H
    # pragma once

    # include <openssl/macros.h>
    # if !OPENSSL_API_3
    #  define HEADER_FILE_H
    # endif
    ...

This is going to ensure that applications which use the old include guards
externally, for example like this

    #ifndef HEADER_FILE_H
    # include <openssl/file.h>
    #endif

will not fail to compile.

In addition to the legacy guard, the public header files also receive a
'# pragma once' directive.

Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/9333)