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.

lib/init.c

@@ -71,6 +71,10 @@ iscsi_create_context(const char *initiator_name)
 	iscsi->use_immediate_data                     = ISCSI_IMMEDIATE_DATA_YES;
 	iscsi->want_header_digest                     = ISCSI_HEADER_DIGEST_NONE_CRC32C;
 
+	iscsi->tcp_keepcnt=3;
+	iscsi->tcp_keepintvl=30;
+	iscsi->tcp_keepidle=30;
+
 	return iscsi;
 }