Prefer build_unibyte_string where applicable

* src/fns.c (syms_of_fns):
* src/image.c (slurp_image): Prefer build_unibyte_string (str) to
make_unibyte_string (str, strlen (str)).
* admin/coccinelle/unibyte_string.cocci: Support string literals.

admin/coccinelle/unibyte_string.cocci

@@ -4,3 +4,9 @@ identifier I;
 @@
 - make_unibyte_string (I, strlen (I))
 + build_unibyte_string (I)
+
+@@
+constant C;
+@@
+- make_unibyte_string (C, strlen (C))
++ build_unibyte_string (C)

src/fns.c

@@ -6337,7 +6337,7 @@ The same variable also affects the function `read-answer'.  See also
   DEFVAR_LISP ("yes-or-no-prompt", Vyes_or_no_prompt,
     doc: /* String to append when `yes-or-no-p' asks a question.
 For best results this should end in a space.  */);
-  Vyes_or_no_prompt = make_unibyte_string ("(yes or no) ", strlen ("(yes or no) "));
+  Vyes_or_no_prompt = build_unibyte_string ("(yes or no) ");
 
   defsubr (&Sidentity);
   defsubr (&Srandom);

src/image.c

@@ -4373,7 +4373,7 @@ slurp_image (Lisp_Object filename, ptrdiff_t *size, const char *image_type)
   char *result = slurp_file (fd, size);
   if (result == NULL)
     image_error ("Error loading %s image `%s'",
-		 make_unibyte_string (image_type, strlen (image_type)),
+		 build_unibyte_string (image_type),
 		 file);
   return result;
 }