Rationalize code placement between wchar.c, encnames.c, and mbutils.c.

Move all the backend-only code that'd crept into wchar.c and encnames.c
into mbutils.c.

To remove the last few #ifdef dependencies from wchar.c and encnames.c,
also make the following changes:

* Adjust get_encoding_name_for_icu to return NULL, not throw an error,
for unsupported encodings.  Its sole caller can perfectly well throw an
error instead.  (While at it, I also made this function and its sibling
is_encoding_supported_by_icu proof against out-of-range encoding IDs.)

* Remove the overlength-name error condition from pg_char_to_encoding.
It's completely silly not to treat that just like any other
the-name-is-not-in-the-table case.

Also, get rid of pg_mic_mblen --- there's no obvious reason why
conv.c shouldn't call pg_mule_mblen instead.

Other than that, this is just code movement and comment-polishing with
no functional changes.  Notably, I reordered declarations in pg_wchar.h
to show which functions are frontend-accessible and which are not.

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

Author: tglsfdc

src/backend/utils/adt/pg_locale.c

@@ -1555,9 +1555,14 @@ init_icu_converter(void)
 	UConverter *conv;
 
 	if (icu_converter)
-		return;
+		return;					/* already done */
 
 	icu_encoding_name = get_encoding_name_for_icu(GetDatabaseEncoding());
+	if (!icu_encoding_name)
+		ereport(ERROR,
+				(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+				 errmsg("encoding \"%s\" not supported by ICU",
+						pg_encoding_to_char(GetDatabaseEncoding()))));
 
 	status = U_ZERO_ERROR;
 	conv = ucnv_open(icu_encoding_name, &status);

src/backend/utils/mb/conv.c

@@ -115,7 +115,7 @@ mic2latin(const unsigned char *mic, unsigned char *p, int len,
 		}
 		else
 		{
-			int			l = pg_mic_mblen(mic);
+			int			l = pg_mule_mblen(mic);
 
 			if (len < l)
 				report_invalid_encoding(PG_MULE_INTERNAL, (const char *) mic,
@@ -217,7 +217,7 @@ mic2latin_with_table(const unsigned char *mic,
 		}
 		else
 		{
-			int			l = pg_mic_mblen(mic);
+			int			l = pg_mule_mblen(mic);
 
 			if (len < l)
 				report_invalid_encoding(PG_MULE_INTERNAL, (const char *) mic,