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 "")

libmysqld/lib_sql.cc

@@ -692,8 +692,7 @@ void *create_embedded_thd(int client_flag)
   thd->client_capabilities= client_flag;
   thd->real_id= pthread_self();
 
-  thd->db= NULL;
-  thd->db_length= 0;
+  thd->db= null_clex_str;
 #ifndef NO_EMBEDDED_ACCESS_CHECKS
   thd->security_ctx->db_access= DB_ACLS;
   thd->security_ctx->master_access= ~NO_ACCESS;