Update from Gnulib

This incorporates:
2020-02-23 use 'restrict'
* lib/careadlinkat.h, lib/md5.h, lib/sha1.h, lib/sha256.h:
* lib/sha512.h, lib/strftime.h, lib/string.in.h, m4/nstrftime.m4:
Copy from Gnulib.
* m4/gnulib-comp.m4: Regenerate.

lib/careadlinkat.h

@@ -47,7 +47,7 @@ struct allocator;
    set errno.  */
 
 char *careadlinkat (int fd, char const *filename,
-                    char *buffer, size_t buffer_size,
+                    char *restrict buffer, size_t buffer_size,
                     struct allocator const *alloc,
                     ssize_t (*preadlinkat) (int, char const *,
                                             char *, size_t));