libc: Restore prior C23 include guards

To avoid any sort of POLA violation, this commit restores old
guards and defines the C23 feature test macros in addition to
them.  This is to close off whole class of possible breakage,
rather than patching it case by case.

Reported by:	dim
Reviewed by:	dim, dteske, fuz
Approved by:	dim, dteske (mentor), fuz (mentor)
MFC after:	1 month
Differential Revision: https://reviews.freebsd.org/D58911

include/assert.h

@@ -68,8 +68,10 @@
 #endif /* __BSD_VISIBLE */
 #endif /* NDEBUG */
 
-#ifndef __STDC_VERSION_ASSERT_H__
-#define __STDC_VERSION_ASSERT_H__ 202311L
+#ifndef _ASSERT_H_
+#define _ASSERT_H_
+
+#define __STDC_VERSION_ASSERT_H__	202311L
 
 /*
  * Static assertions.  In principle we could define static_assert for
@@ -92,4 +94,4 @@ __BEGIN_DECLS
 void __assert(const char *, const char *, int, const char *) __dead2;
 __END_DECLS
 
-#endif /* !__STDC_VERSION_ASSERT_H__ */
+#endif /* !_ASSERT_H_ */

include/limits.h

@@ -29,7 +29,9 @@
  * SUCH DAMAGE.
  */
 
-#ifndef __STDC_VERSION_LIMITS_H__
+#ifndef _LIMITS_H_
+#define _LIMITS_H_
+
 #define __STDC_VERSION_LIMITS_H__	202311L
 
 #include <sys/cdefs.h>
@@ -148,4 +150,4 @@
 #include <sys/syslimits.h>
 #endif
 
-#endif /* !__STDC_VERSION_LIMITS_H__ */
+#endif /* !_LIMITS_H_ */

include/rpc/xdr.h

@@ -332,7 +332,7 @@ __BEGIN_DECLS
 extern void   xdrmem_create(XDR *, char *, u_int, enum xdr_op);
 
 /* XDR using stdio library */
-#ifdef __STDC_VERSION_STDIO_H__
+#ifdef _STDIO_H_
 extern void   xdrstdio_create(XDR *, FILE *, enum xdr_op);
 #endif
 

include/stdio.h

@@ -32,7 +32,9 @@
  * SUCH DAMAGE.
  */
 
-#ifndef __STDC_VERSION_STDIO_H__
+#ifndef	_STDIO_H_
+#define	_STDIO_H_
+
 #define __STDC_VERSION_STDIO_H__	202311L
 
 #include <sys/cdefs.h>
@@ -540,4 +542,4 @@ extern int __isthreaded;
 __END_DECLS
 __NULLABILITY_PRAGMA_POP
 
-#endif /* !__STDC_VERSION_STDIO_H__ */
+#endif /* !_STDIO_H_ */

include/xlocale.h

@@ -68,7 +68,7 @@ __BEGIN_DECLS
 #include <xlocale/_ctype.h>
 #endif
 
-#ifdef __STDC_VERSION_STDIO_H__
+#ifdef _STDIO_H_
 #include <xlocale/_stdio.h>
 #endif