rename internally used client API to avoid name conflicts with C/C

MariaDB server has remnants of libmysqlclient used internally (for replicaiton, CONNECT, Federated, Spider, etc).

Connector/C has many symbols with exactly same names, so if a server would load libmariadb.so, directly or indirectly, one can get symbols mixed up, causing problems.

A possible scenario — server loads CONNECT engine (ha_connect.so), the latter uses ODBC to connect to another MariaDB server. ODBC library loads MariaDB Connector/ODBC driver, and that driver is dynamically linked with libmariadb.so. A possible outcome — ODBC could use mysql_connect from libmariadb.so to connect, but mysql_close from the server to disconnect. Or vice versa.

To avoid these problems the internal server implementation of the client API should be renamed.