Unable to configure external locking in MariaDB with --skip-external-locking

Description

In this documentation: https://mariadb.com/kb/en/server-system-variables/#skip_external_locking,
the option "--skip-external-locking" can be used to configure mariadb to use
external locking or not. However, this option takes no effect when specified
in the "/etc/my.cnf" configuration file.

How to repeat

Install MariaDB server:

sudo apt install mariadb-server

Shut down the MariaDB server:

sudo systemctl stop mariadb.service

Edit the configuration file "/etc/my.cnf" with the following content:

[mariadb]

skip-external-locking=OFF

Such that we don't skip external locking, i.e., we use external locking.

Restart MariaDB server:

sudo systemctl start mariadb.service

Export the system variables, and observe that external-locking is still ON:

sudo mariadb -u root -p -e "SHOW VARIABLES LIKE 'skip_external_locking';"

The root cause seems to be that, in line 3164 of commit 3a06964 for mariadb/server
repo, "--skip-external-locking" seems to be set as a read-only variable with
default value of true, making it immutable.

Suggested fix

The documentation of mysql server states that the variable can be configured.
However, the source code says otherwise. A potential fix is to clarify whether
this option is configurable and reject the invalid value if it does not support
value assignment.