Changed database, tablename and alias to be LEX_CSTRING

This was done in, among other things:
- thd->db and thd->db_length
- TABLE_LIST tablename, db, alias and schema_name
- Audit plugin database name
- lex->db
- All db and table names in Alter_table_ctx
- st_select_lex db

Other things:
- Changed a lot of functions to take const LEX_CSTRING* as argument
  for db, table_name and alias. See init_one_table() as an example.
- Changed some function arguments from LEX_CSTRING to const LEX_CSTRING
- Changed some lists from LEX_STRING to LEX_CSTRING
- threads_mysql.result changed because process list_db wasn't always
  correctly updated
- New append_identifier() function that takes LEX_CSTRING* as arguments
- Added new element tmp_buff to Alter_table_ctx to separate temp name
  handling from temporary space
- Ensure we store the length after my_casedn_str() of table/db names
- Removed not used version of rename_table_in_stat_tables()
- Changed Natural_join_column::table_name and db_name() to never return
  NULL (used for print)
- thd->get_db() now returns db as a printable string (thd->db.str or "")

include/mysql/service_my_print_error.h

@@ -50,11 +50,12 @@ extern struct my_print_error_service_st {
 #define my_printv_error(A,B,C,D) my_print_error_service->my_printv_error_func(A,B,C,D)
 
 #else
-
+#ifndef MY_ERROR_DEFINED
 extern void my_error(unsigned int nr, unsigned long MyFlags, ...);
 extern void my_printf_error(unsigned int my_err, const char *format, unsigned long MyFlags, ...);
 extern void my_printv_error(unsigned int error, const char *format, unsigned long MyFlags,va_list ap);
-#endif
+#endif /* MY_ERROR_DEFINED */
+#endif /* MYSQL_DYNAMIC_PLUGIN */
 
 #ifdef __cplusplus
 }