[cmyth] Use CLIENT_FOUND_ROWS option

Setting this option makes mysql_affected_rows return
the number of rows found, or better number of rows
matching the WHERE clause.

This fixes functions that return mysql_affected_rows
as error indicator. Those functions would return an
error if an UPDATE query would reset an already
existing value.

Affected are:
cmyth_mysql_set_watched_status
cmyth_mysql_update_bookmark_setting
cmyth_mysql_update_recordingrule