Update header comments for wchar.c and encnames.c.

Bring these into common style (including having proper copyright
notices) and adjust their self-declaration of where they live.

Discussion: https://postgr.es/m/CA+TgmoYO8oq-iy8E02rD8eX25T-9SmyxKWqqks5OMHxKvGXpXQ@mail.gmail.com

src/common/encnames.c

-/*
- * Encoding names and routines for work with it. All
- * in this file is shared between FE and BE.
+/*-------------------------------------------------------------------------
+ *
+ * encnames.c
+ *	  Encoding names and routines for working with them.
+ *
+ * Portions Copyright (c) 2001-2020, PostgreSQL Global Development Group
+ *
+ * IDENTIFICATION
+ *	  src/common/encnames.c
  *
- * src/backend/utils/mb/encnames.c
+ *-------------------------------------------------------------------------
  */
 #ifdef FRONTEND
 #include "postgres_fe.h"

src/common/wchar.c

-/*
- * conversion functions between pg_wchar and multibyte streams.
- * Tatsuo Ishii
- * src/backend/utils/mb/wchar.c
+/*-------------------------------------------------------------------------
+ *
+ * wchar.c
+ *	  Functions for working with multibyte characters in various encodings.
+ *
+ * Portions Copyright (c) 1998-2020, PostgreSQL Global Development Group
+ *
+ * IDENTIFICATION
+ *	  src/common/wchar.c
  *
+ *-------------------------------------------------------------------------
  */
-/* can be used in either frontend or backend */
 #ifdef FRONTEND
 #include "postgres_fe.h"
 #else