Simplify caseup() and casedn() in charsets

After the MDEV-13118 fix there's no code in the server that
wants caseup/casedn to change the argument in place for simple
charsets.  Let's remove this logic and always return the result in a
new string for all charsets, both simple and complex.

1. Removing the optimization that *some* character sets used in casedn()
  and caseup(), which allowed (and required) to change the case in-place,
  overwriting the string passed as the "src" argument.
  Now all CHARSET_INFO's work in the same way:
  non of them change the source string in-place, all of them now convert
  case from the source string to the destination string, leaving
  the source string untouched.

2. Adding "const" qualifier to the "char *src" parameter
   to caseup() and casedn().

3. Removing duplicate implementations in ctype-mb.c.
  Now both caseup() and casedn() implementations for all CJK character sets
  use internally the same function my_casefold_mb()
  (the former my_casefold_mb_varlen()).

4. Removing the "unused" attribute from parameters of some my_case{up|dn}_xxx()
   implementations, as the affected parameters are now *used* in the code.
   Previously these parameters were used only in DBUG_ASSERT().

strings/ctype-bin.c

@@ -220,11 +220,11 @@ static size_t my_case_str_bin(CHARSET_INFO *cs __attribute__((unused)),
 
 
 static size_t my_case_bin(CHARSET_INFO *cs __attribute__((unused)),
-                          char *src __attribute__((unused)),
-                          size_t srclen,
-                          char *dst __attribute__((unused)),
-                          size_t dstlen __attribute__((unused)))
+                          const char *src, size_t srclen,
+                          char *dst, size_t dstlen)
 {
+  DBUG_ASSERT(srclen <= dstlen);
+  memcpy(dst, src, srclen);
   return srclen;
 }