Add setters for TCP keepalive values

This patch adds 3 functions to set the 3 keepalive values TCP_KEEPIDLE, TCP_KEEPCNT
and TCP_KEEPINTVL. The values have to be set after iscsi context creation and are
then configured on the socket on each new connection.

include/iscsi-private.h

@@ -86,6 +86,9 @@ struct iscsi_context {
 	int is_connected;
 	
 	int tcp_user_timeout;
+	int tcp_keepcnt;
+	int tcp_keepintvl;
+	int tcp_keepidle;
 
 	int current_phase;
 	int next_phase;