MDEV-4660 SHUTDOWN command

Based on James Briggs contribution.

mysql-test/r/shutdown.result

@@ -0,0 +1,5 @@
+create user user1@localhost;
+shutdown;
+ERROR 42000: Access denied; you need (at least one of) the SHUTDOWN privilege(s) for this operation
+shutdown;
+drop user user1@localhost;

mysql-test/t/shutdown.test

@@ -0,0 +1,28 @@
+--source include/not_embedded.inc
+#
+# SHUTDOWN statement
+#
+create user user1@localhost;
+
+connect (c1,localhost,user1,,);
+
+--error ER_SPECIFIC_ACCESS_DENIED_ERROR
+shutdown;
+
+connection default;
+disconnect c1;
+
+--let $_server_id= `SELECT @@server_id`
+--let $_expect_file_name= $MYSQLTEST_VARDIR/tmp/mysqld.$_server_id.expect
+--exec echo "wait" > $_expect_file_name
+
+shutdown;
+
+--source include/wait_until_disconnected.inc
+
+--exec echo "restart" > $_expect_file_name
+--enable_reconnect
+--source include/wait_until_connected_again.inc
+
+drop user user1@localhost;
+