Add option --enable-cleartext-plugin to the MariaDB client

For compatibility reasons, add the option to the MariaDB client without
any functional changes besides simply accepting the option and emitting
a warning that it is obsolete.

In MySQL this security related option is compulsory in certain use
cases. When users switch to MariaDB, this client command that used to
work starts failing without a sensible error message. In worst case
users resort to re-installing the mysql client from MySQL.

In MariaDB the option is obsolete and should simply be ignored. Users
however don't have any opportunity to learn that unless the client
program tells them so.

Before:

    mysql --enable-cleartext-plugin ...
    mysql: unknown option '--enable-cleartext-plugin'

    (program terminates)

After:

    mysql --enable-cleartext-plugin ...
    WARNING: option '--enable-cleartext-plugin' is obsolete.

    (program executes)

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.

mysql-test/main/mysql.test

@@ -708,3 +708,11 @@ drop table t1;
 --echo # MDEV-15538 '-N' Produce html output wrong
 --echo #
 --exec $MYSQL -NHe "select 1 as a"
+
+
+#
+# Test obsolete option --enable-cleartext-plugin
+# This should proceed with a warning
+#
+--echo
+--exec $MYSQL test --enable-cleartext-plugin -e "select 1"