source3/lib/server_prefork.c set socket close on exec

Set SOCKET_CLOEXEC on the sockets returned by accept.  This ensures that
the socket is unavailable to any child process created by system().
Making it harder for malicious code to set up a command channel,
as seen in the exploit for CVE-2015-0240

Signed-off-by: Gary Lockyer <gary@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>

source3/lib/server_prefork.c

@@ -620,6 +620,7 @@ static void prefork_listen_accept_handler(struct tevent_context *ev,
 			  state->error, strerror(state->error)));
 		goto done;
 	}
+	smb_set_close_on_exec(sd);
 
 	state->accept_fd = sd;