stdio: Support several gnulib-tool invocations better. * lib/stdio.in.h: Test _GL_SKIP_GNULIB_STDIO_H. (_GL_ALREADY_INCLUDING_STDIO_H): Rename to a macro that depends on GUARD_PREFIX. * lib/fopen.c: Set _GL_SKIP_GNULIB_STDIO_H instead of _GL_ALREADY_INCLUDING_STDIO_H. * lib/freopen.c: Likewise.