MDEV-7947 strcmp() takes 0.37% in OLTP RO

This patch ensures that all identical character sets shares the same
cs->csname.
This allows us to replace strcmp() in my_charset_same() with comparisons
of pointers. This fixes a long standing performance issue that could cause
as strcmp() for every item sent trough the protocol class to the end user.

One consequence of this patch is that we don't allow one to add a character
definition in the Index.xml file that changes the csname of an existing
character set. This is by design as changing character set names of existing
ones is extremely dangerous, especially as some storage engines just records
character set numbers.

As we now have a hash over character set's csname, we can in the future
use that for faster access to a specific character set. This could be done
by changing the hash to non unique and use the hash to find the next
character set with same csname.

include/mysys_err.h

@@ -72,7 +72,8 @@ extern const char *globerrs[];  /* my_error_messages is here */
 #define EE_BADMEMORYRELEASE     36
 #define EE_PERM_LOCK_MEMORY     37
 #define EE_MEMCNTL              38
-#define EE_ERROR_LAST           38 /* Copy last error nr */
+#define EE_DUPLICATE_CHARSET    39
+#define EE_ERROR_LAST           39 /* Copy last error nr */
 
 /* Add error numbers before EE_ERROR_LAST and change it accordingly. */