Make better Unicode-compliant and unify some UTF-8 code pathes.

* grub-core/normal/charset.c (grub_utf8_to_utf16): Don't eat possibly
	valid character.
	(grub_is_valid_utf8): Use grub_utf8_process.
	Check resulting code range.
	(grub_utf8_to_ucs4): Use grub_utf8_process.
	* include/grub/charset.h (grub_utf16_to_utf8): Don't eat up a possibly
	valid character.

include/grub/charset.h

@@ -76,6 +76,8 @@ grub_utf16_to_utf8 (grub_uint8_t *dest, const grub_uint16_t *src,
 	    {
 	      /* Error...  */
 	      *dest++ = '?';
+	      /* *src may be valid. Don't eat it.  */
+	      src--;
 	    }
 
 	  code_high = 0;