Add Event Forwarding Interfaces to switch daemon/manager

This patch will add an interface to redirect switch related events on a running switch daemon/manager. 

* C API ([trema]/src/lib/event_fwd_interface.h)
* CLI ([trema]/examples/switch_event_cfg/{add_fwd_entry, delete_fwd_entry, set_fwd_entry, dump_fwd_entry})
    * See switch_event_cfg.feature for usage detail.

By using these APIs user defined controller can start listening on switch events without writing `event` line on trema network DSL.