Handle several functions now being provided by glibc

We mention that these are now superseded by the glibc implementations,
make the headers cope with already declared functions on glibc-based
systems, and document this in the man pages.

include/bsd/string.h

@@ -42,7 +42,9 @@ size_t strlcat(char *dst, const char *src, size_t siz);
 char *strnstr(const char *str, const char *find, size_t str_len);
 void strmode(mode_t mode, char *str);
 
+#if defined(_GNU_SOURCE) && defined(__GLIBC__) && !__GLIBC_PREREQ(2, 25)
 void explicit_bzero(void *buf, size_t len);
+#endif
 __END_DECLS
 
 #endif