MDEV-17658 change the structure of mysql.user table

Implement User_table_json.
Fix scripts to use mysql.global_priv.
Fix tests.

mysql-test/main/view_grant.test

@@ -669,16 +669,6 @@ drop database mysqltest;
 #
 # Bug#14256 definer in view definition is not fully qualified
 #
---disable_warnings
-drop view if exists v1;
-drop table if exists t1;
---enable_warnings
-
-# Backup anonymous users and remove them. (They get in the way of
-# the one we test with here otherwise.)
-create table t1 as select * from mysql.user where user='';
-delete from mysql.user where user='';
-flush privileges;
 
 # Create the test user
 create user 'test14256'@'%';
@@ -708,12 +698,6 @@ connection root;
 disconnect test14256;
 drop user test14256;
 
-# Restore the anonymous users.
-insert into mysql.user select * from t1;
-flush privileges;
-
-drop table t1;
-
 #
 # Bug#14726 freeing stack variable in case of an error of opening a view when
 #           we have locked tables with LOCK TABLES statement.