Allow extra arguments for database command line tools in kea-admin

The idea is to allow multiple -x / --extra to kea-admin: these extra arguments are passed to the database command line tool, e.g. -x --ssl will pass --ssl to mysql when the user managing a MySQL database was configured to require SSL/TLS protection of the connection to the server.

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information