ustcomm: implement shutdown API

Implement socket shutdown API to allow the ust-ctl library to shutdown
the communication socket when it cannot recover from errors which make
the sender and receiver out-of-sync.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Change-Id: If896396d9104b21aee09981cac773996ed79aa4d