Fix lttcomm_close_unix_sock to actually close the socket

To all soul out there, shutdown(sock, SHUT_RDWR) does NOT close the
socket keeping it open after the call. I have to say for my defense that
the man page does not specify it clearly :P.

Reported-by: Irina Guilman <irina.guilman@ericsson.com>
Signed-off-by: David Goulet <dgoulet@efficios.com>