MDEV-21375: Get option group suffix from $MARIADB_GROUP_SUFFIX in addition to $MYSQL_GROUP_SUFFIX

[amarjilani]

Description

Currently, if the --default-group-suffix argument is not passed when initializing
an instance of MariaDB, the MYSQL_GROUP_SUFFIX environment variable, if set, is used to specify the group suffix.

Add an additional check for the MARIADB_GROUP_SUFFIX environment variable before checking for MYSQL_GROUP_SUFFIX. This environment variable will take precedence over the MYSQL_GROUP_SUFFIX if both are set, with the goal of furthering MariaDB's branding.

This behaviour is consistent with how MARIADB_HOME is checked before MYSQL_HOME (MDEV-21365).

Release Notes

N/A

How can this PR be tested?

Create a custom config file with at least two different suffix groups.
For example:
test.cnf

[mysqld]
max_connections=100

[mysqld_test1]
max_connections=300

[mysqld_test2]
max_connections=500

Set the MARIADB_GROUP_SUFFIX and MYSQL_GROUP_SUFFIX environment variables:

export MARIADB_GROUP_SUFFIX=_test1
export MYSQL_GROUP_SUFFIX=_test2

Start the server, passing in the config file:

./build/sql/mariadbd --defaults-file=./my-test.cnf --datadir=./data --user=username &

Run the client:

./build/client/mariadb -u root

Run the appropriate query:

MariaDB [(none)]> SHOW VARIABLES LIKE 'max_connections';
+-----------------+-------+
| Variable_name   | Value |
+-----------------+-------+
| max_connections | 300   |
+-----------------+-------+

Basing the PR against the correct MariaDB version

• This is a new feature and the PR is based against the latest MariaDB development branch.

Copyright

All new code of the whole pull request, including one or several files that are
either new files or modified ones, are contributed under the BSD-new license. I
am contributing on behalf of my employer Amazon Web Services, Inc.

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