Expand on API guarantee for fgetln

OpenBSD has begun using the API guarantee that *len is not 0 if
the fgetln(3) return value is not NULL; document this explicitly
in the manpage and add a comment to the function implementation
that this doesn’t need to be checked because getdelim/getline have
similar guarantees.

Signed-off-by: Thorsten Glaser <tg@mirbsd.org>

src/fgetln.c

@@ -39,6 +39,7 @@ fgetln (FILE *stream, size_t *len)
 	ssize_t nread;
 
 	nread = getline(&line, &line_len, stream);
+	/* Note: the getdelim/getline API ensures nread != 0. */
 	if (nread == -1) {
 		*len = 0;
 		return NULL;