Add iscsi_tcp_set_user_timeout function

This patch adds a user configurable option to set the TCP_USER_TIMEOUT
socket option. With this timeout set a broken TCP session is shutdown
after a given timeout even there are unacked packets. SO_KEEPALIVE
seems not to work in this case.

include/iscsi-private.h

@@ -84,6 +84,8 @@ struct iscsi_context {
 
 	int fd;
 	int is_connected;
+	
+	int tcp_user_timeout;
 
 	int current_phase;
 	int next_phase;