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/libiscsi.def

@@ -73,6 +73,9 @@ iscsi_set_session_type
 iscsi_set_targetname
 iscsi_set_tcp_keepalive
 iscsi_set_tcp_user_timeout
+iscsi_set_tcp_keepidle
+iscsi_set_tcp_keepcnt
+iscsi_set_tcp_keepintvl
 iscsi_startstopunit_sync
 iscsi_startstopunit_task
 iscsi_synchronizecache10_sync