Can't create federated table in MariaDB using CREATE SERVER - 'Foreign data src error'
Nuts.  I'd been looking at the documentation on the MySQL web site for version 5.5 of MySQL.

When I looked up the MariaDB's site and explored its knowledge base pages I discovered that:

a.  They've abandoned the FEDERATED engined and have replaced it with a FederatedX engine, even though:

b.  The syntax still uses the 'FEDERATED' key word, but also

c.  They have added a 'Socket' option to the CREATE SERVER statement.

The 'Socket' column was in the listing of the mysql.server table but I assumed it should just be blank, and it wasn't mentioned in the MySQL documentation.

Anyway, adding a 'socket' option to CREATE SERVER to tell MariaDB to explicitly user the database server's socket has solved the problem.

Weird how the SOCKET wasn't necessary in the long-form CONNECT string though.