The result of show variables is not compatible with MySQL

[zimulala]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. What did you do?
   If possible, provide a recipe for reproducing the error.

select @@global.sql_log_bin;
show variables like "sql_log_bin";

2. What did you expect to see?
   Your MySQL connection id is 172
   Server version: 5.7.20 Homebrew

mysql> select @@global.sql_log_bin;
+----------------------+
| @@global.sql_log_bin |
+----------------------+
|                    1 |
+----------------------+
1 row in set, 1 warning (0.00 sec)

mysql> show variables like "sql_log_bin";
+---------------+-------+
| Variable_name | Value |
+---------------+-------+
| sql_log_bin   | ON    |
+---------------+-------+
1 row in set (0.00 sec)

3. What did you see instead?

tidb> select @@global.sql_log_bin;
+----------------------+
| @@global.sql_log_bin |
+----------------------+
| 1                    |
+----------------------+
1 row in set (0.00 sec)

tidb> show variables like "sql_log_bin";
+---------------+-------+
| Variable_name | Value |
+---------------+-------+
| sql_log_bin   | 1     |
+---------------+-------+
1 row in set (0.02 sec)

4. What version of TiDB are you using (tidb-server -V or run select tidb_version(); on TiDB)?
   Release Version: v3.0.0-beta-69-g84d1513b0-dirty
   Git Commit Hash: 84d1513
   Git Branch: master

[ghost]

This is a duplicate of #15131

It requires a type system for system variables, so that depending on context of @@ or show variables the value can be returned correctly.

I am going to close this to reduce duplication.