chars: represent the high-bit controls more intelligibly

Instead of showing the upper control codes like this:

   ^À ^Á ^Â ^Ã ^Ä ^Å ^Æ ^Ç ^È ^É ^Ê ^Ë ^Ì ^Í ^Î ^Ï
   ^Ð ^Ñ ^Ò ^Ó ^Ô ^Õ ^Ö ^× ^Ø ^Ù ^Ú ^Û ^Ü ^Ý ^Þ ^ß

show them like this:

   ^` ^a ^b ^c ^d ^e ^f ^g ^h ^i ^j ^k ^l ^m ^n ^o
   ^p ^q ^r ^s ^t ^u ^v ^w ^x ^y ^z ^{ ^| ^} ^~ ^=

The lower control codes continue to be shown like this:

   ^@ ^A ^B ^C ^D ^E ^F ^G ^H ^I ^J ^K ^L ^M ^N ^O
   ^P ^Q ^R ^S ^T ^U ^V ^W ^X ^Y ^Z ^[ ^\ ^] ^^ ^_

The representation of DEL (0x7F) continues as ^?.

Further, use knowledge of UTF-8 to avoid a roundtrip through
wide characters.

src/chars.c

@@ -219,7 +219,7 @@ bool is_word_mbchar(const char *c, bool allow_punct)
 
 /* c is a control character.  It displays as ^@, ^?, or ^[ch], where ch
  * is (c + 64).  We return that character. */
-char control_rep(char c)
+char control_rep(const signed char c)
 {
     assert(is_cntrl_char(c));
 
@@ -228,6 +228,10 @@ char control_rep(char c)
 	return '@';
     else if (c == NANO_CONTROL_8)
 	return '?';
+    else if (c == -97)
+	return '=';
+    else if (c < 0)
+	return c + 224;
     else
 	return c + 64;
 }
@@ -250,17 +254,18 @@ wchar_t control_wrep(wchar_t wc)
 #endif
 
 /* c is a multibyte control character.  It displays as ^@, ^?, or ^[ch],
- * where ch is (c + 64).  We return that multibyte character. */
+ * where ch is (c + 64).  We return that single-byte character. */
 char *control_mbrep(const char *c, char *crep, int *crep_len)
 {
     assert(c != NULL && crep != NULL && crep_len != NULL);
 
 #ifdef ENABLE_UTF8
     if (use_utf8) {
-	wchar_t wc;
-
-	IGNORE_CALL_RESULT(mbtowc(&wc, c, MB_CUR_MAX));
-	*crep_len = wctomb(crep, control_wrep(wc));
+	if (0 <= c[0] && c[0] <= 127)
+	    *crep = control_rep(c[0]);
+	else
+	    *crep = control_rep(c[1]);
+	*crep_len = 1;
     } else
 #endif
     {

src/proto.h

@@ -189,7 +189,7 @@ bool is_cntrl_wchar(wchar_t wc);
 bool is_cntrl_mbchar(const char *c);
 bool is_punct_mbchar(const char *c);
 bool is_word_mbchar(const char *c, bool allow_punct);
-char control_rep(char c);
+char control_rep(const signed char c);
 #ifdef ENABLE_UTF8
 wchar_t control_wrep(wchar_t wc);
 #endif