libc: Implement qualifier-preserving standard library functions

Several standard library functions are specified to return an unqualified
pointer while accepting a pointer to a potentially const-qualified object.
N3020 addresses this behaviour, discarding qualifiers due to incompatible
pointer types, by introducing qualifier-preserving macros for the affected
set of standard library functions.

Add `__qualsel()` helper to `<sys/cdefs.h>`, implemented using the generic
selection, and define qualifier-preserving macros for that set of functions
in `<string.h>`, `<wchar.h>`, and `<stdlib.h>`.

Macros are gated on `_STDC_VERSION__ >= 202311L && !__cplusplus`, therefore
there is no behavioural change for earlier C modes or C++ translation units.
The kernel is likewise unaffected, as it does not include userland headers.

As function-like macros, they are transparent except at a call site where
the address-of operator is applied, the macro is suppressed via `#undef`,
or the identifier appears in parenthesised form; all of which cause the
underlying function designator to be used instead.

Reviewed by:	fuz
Approved by:	fuz (mentor)
MFC after:	1 month
Pull Request:	https://github.com/freebsd/freebsd-src/pull/2288

include/stdlib.h

@@ -418,4 +418,16 @@ errno_t	 qsort_s(void *, rsize_t, rsize_t,
 __END_DECLS
 __NULLABILITY_PRAGMA_POP
 
+#if defined(__qualsel) && !defined(__cplusplus) && \
+    defined(__STDC_VERSION__) && __STDC_VERSION__ >= 202311L
+#define	bsearch(key, base, nmemb, size, compar)		__qualsel((base),    \
+	(const void *)(bsearch)((key), (base), (nmemb), (size), (compar)),   \
+	(bsearch)((key), (base), (nmemb), (size), (compar)))
+#ifdef __BLOCKS__
+#define	bsearch_b(key, base, nmemb, size, compar)	__qualsel((base),    \
+	(const void *)(bsearch_b)((key), (base), (nmemb), (size), (compar)), \
+	(bsearch_b)((key), (base), (nmemb), (size), (compar)))
+#endif
+#endif
+
 #endif /* !_STDLIB_H_ */

include/string.h

@@ -205,4 +205,32 @@ errno_t memset_s(void *, rsize_t, int, rsize_t);
 #endif /* __EXT1_VISIBLE */
 __END_DECLS
 
+#if defined(__qualsel) && !defined(__cplusplus) && \
+    defined(__STDC_VERSION__) && __STDC_VERSION__ >= 202311L
+#define	memchr(b, c, n)		__qualsel((b),				\
+	(const void *)(memchr)((b), (c), (n)), (memchr)((b), (c), (n)))
+#define	strchr(s, c)		__qualsel((s),				\
+	(const char *)(strchr)((s), (c)), (strchr)((s), (c)))
+#define	strpbrk(s, charset)	__qualsel((s),				\
+	(const char *)(strpbrk)((s), (charset)), (strpbrk)((s), (charset)))
+#define	strrchr(s, c)		__qualsel((s),				\
+	(const char *)(strrchr)((s), (c)), (strrchr)((s), (c)))
+#define	strstr(s, find)		__qualsel((s),				\
+	(const char *)(strstr)((s), (find)), (strstr)((s), (find)))
+#if __BSD_VISIBLE
+#define	memmem(b, blen, pat, plen)	__qualsel((b),			\
+	(const void *)(memmem)((b), (blen), (pat), (plen)),		\
+	(memmem)((b), (blen), (pat), (plen)))
+#define	memrchr(b, c, n)	__qualsel((b),				\
+	(const void *)(memrchr)((b), (c), (n)), (memrchr)((b), (c), (n)))
+#define	strcasestr(s, find)	__qualsel((s),				\
+	(const char *)(strcasestr)((s), (find)), (strcasestr)((s), (find)))
+#define	strchrnul(s, c)		__qualsel((s),				\
+	(const char *)(strchrnul)((s), (c)), (strchrnul)((s), (c)))
+#define	strnstr(s, find, slen)	__qualsel((s),				\
+	(const char *)(strnstr)((s), (find), (slen)),			\
+	(strnstr)((s), (find), (slen)))
+#endif /* __BSD_VISIBLE */
+#endif
+
 #endif /* _STRING_H_ */

include/wchar.h

@@ -246,4 +246,18 @@ size_t	wcslcpy(wchar_t *, const wchar_t *, size_t);
 #endif
 __END_DECLS
 
+#if defined(__qualsel) && !defined(__cplusplus) && \
+    defined(__STDC_VERSION__) && __STDC_VERSION__ >= 202311L
+#define	wcschr(s, c)		__qualsel((s),				\
+	(const wchar_t *)(wcschr)((s), (c)), (wcschr)((s), (c)))
+#define	wcspbrk(s, set)		__qualsel((s),				\
+	(const wchar_t *)(wcspbrk)((s), (set)), (wcspbrk)((s), (set)))
+#define	wcsrchr(s, c)		__qualsel((s),				\
+	(const wchar_t *)(wcsrchr)((s), (c)), (wcsrchr)((s), (c)))
+#define	wcsstr(s, find)		__qualsel((s),				\
+	(const wchar_t *)(wcsstr)((s), (find)), (wcsstr)((s), (find)))
+#define	wmemchr(s, c, n)	__qualsel((s),				\
+	(const wchar_t *)(wmemchr)((s), (c), (n)), (wmemchr)((s), (c), (n)))
+#endif
+
 #endif /* !_WCHAR_H_ */