lib/: Include <gshadow.h> if it's available

lib/: Include <gshadow.h> if it's available

The existing code was assuming that libc's <shadow.h> includes
<gshadow.h>.  That's not true.

	alx@debian:~$ find /usr/include/shadow.h
	/usr/include/shadow.h
	alx@debian:~$ find /usr/include/gshadow.h
	/usr/include/gshadow.h
	alx@debian:~$ grep include.*gshadow /usr/include/shadow.h
	alx@debian:~$

As a result, we were unconditionally including our own "gshadow_.h".

Fix that incorrect assumption, and do the following instead:

-  Include unconditionally our own "gshadow_.h".
-  Make our "gshadow_.h" include <gshadow.h> if it exists,
   and only provide the declarations otherwise.

While at it, fix the include guard to be consistent with the project.

Reviewed-by: Serge Hallyn <serge@hallyn.com>
Signed-off-by: Alejandro Colomar <alx@kernel.org>