Bug#57306 SHOW PROCESSLIST does not display string literals well.

Problem: Extended characters outside of ASCII range where not displayed
properly in SHOW PROCESSLIST, because thd_info->query was always sent as 
system_character_set (utf8). This was wrong, because query buffer
is never converted to utf8 - it is always have client character set.

Fix: sending query buffer using query character set

  @ sql/sql_class.cc
  @ sql/sql_class.h
    Introducing a new class CSET_STRING, a LEX_STRING with character set.
    Adding set_query(&CSET_STRING)
    Adding reset_query(), to use instead of set_query(0, NULL).

  @ sql/event_data_objects.cc
    Using reset_query()

  @ sql/log_event.cc
    Using reset_query()
    Adding charset argument to set_query_and_id().

  @ sql/slave.cc
    Using reset_query().

  @ sql/sp_head.cc
    Changing backing up and restore code to use CSET_STRING.

  @ sql/sql_audit.h
    Using CSET_STRING.
    In the "else" branch it's OK not to use
    global_system_variables.character_set_client.
    &my_charset_latin1, which is set in constructor, is fine
    (verified with Sergey Vojtovich).

  @ sql/sql_insert.cc
    Using set_query() with proper character set: table_name is utf8.

  @ sql/sql_parse.cc
    Adding character set argument to set_query_and_id().
    (This is the main point where thd->charset() is stored
     into thd->query_string.cs, for use in "SHOW PROCESSLIST".)
    Using reset_query().
    
  @ sql/sql_prepare.cc
    Storing client character set into thd->query_string.cs.

  @ sql/sql_show.cc
    Using CSET_STRING to fetch and send charset-aware query information
    from threads.

  @ storage/myisam/ha_myisam.cc
    Using set_query() with proper character set: table_name is utf8.

  @ mysql-test/r/show_check.result
  @ mysql-test/t/show_check.test
    Adding tests

sql/event_data_objects.cc

@@ -1526,7 +1526,7 @@ end:
   thd->end_statement();
   thd->cleanup_after_query();
   /* Avoid races with SHOW PROCESSLIST */
-  thd->set_query(NULL, 0);
+  thd->reset_query();
 
   DBUG_PRINT("info", ("EXECUTED %s.%s  ret: %d", dbname.str, name.str, ret));