MDEV-36425 Extend read_only to also block share locks and super user

The main purpose of this allow one to use the --read-only
option to ensure that no one can issue a query that can
block replication.

The --read-only option can now take 4 different values:
0  No read only (as before).
1  Blocks changes for users without the 'READ ONLY ADMIN'
   privilege (as before).
2  Blocks in addition LOCK TABLES and SELECT IN SHARE MODE
   for not 'READ ONLY ADMIN' users.
3  Blocks in addition 'READ_ONLY_ADMIN' users for all the
   previous statements.

read_only is changed to an enum and one can use the following
names for the lock levels:
OFF, ON, NO_LOCK, NO_LOCK_NO_ADMIN

Too keep things compatible with older versions config files, one can
still use values FALSE and TRUE, which are mapped to OFF and ON.

The main visible changes are:
- 'show variables like "read_only"' now returns a string
   instead of a number.
- Error messages related to read_only violations now contains
  the current value off readonly.

Other things:
- is_read_only_ctx() renamed to check_read_only_with_error()
- Moved TL_READ_SKIP_LOCKED to it's logical place

Reviewed by: Sergei Golubchik <serg@mariadb.org>

mysql-test/suite/sql_sequence/gtid-slave.opt

@@ -1,4 +1,4 @@
 --binlog_format=row
 --query_cache_type=1
---read_only=true
+--read_only=ON
 --log-slave-updates

mysql-test/suite/sql_sequence/gtid.result

@@ -239,7 +239,7 @@ show global variables like 'read_only';
 Variable_name	Value
 read_only	ON
 select next value for s_db.s1;
-ERROR HY000: The MariaDB server is running with the --read-only option so it cannot execute this statement
+ERROR HY000: The MariaDB server is running with the --read-only=ON option so it cannot execute this statement
 connection m_normal_1;
 drop sequence s_db.s1;
 ###########################################

mysql-test/suite/sql_sequence/replication-slave.opt

@@ -1 +1 @@
---binlog_format=row --query_cache_type=1 --read_only=true
+--binlog_format=row --query_cache_type=1 --read_only=on

mysql-test/suite/sql_sequence/replication.result

@@ -347,7 +347,7 @@ show global variables like 'read_only';
 Variable_name	Value
 read_only	ON
 select next value for s_db.s1;
-ERROR HY000: The MariaDB server is running with the --read-only option so it cannot execute this statement
+ERROR HY000: The MariaDB server is running with the --read-only=ON option so it cannot execute this statement
 connection m_normal_1;
 drop sequence s_db.s1;
 ###########################################