NOP count in flight of client generated NOP-Outs

This patch adds the abilitiy to libiscsi to count the number
of consecutive outstanding NOPs.

With this ability its fairly easy to implement a keepalive
check with NOPs in your application.

Periodically (e.g. every 5 secs) create a NOP-Out with:

iscsi_nop_out_async(iscsi, NULL, NULL, 0, NULL);

At that time check the number of consecutive missing NOP-Ins
with

iscsi_get_nops_in_flight(iscsi) > N.

Where N is the number of missing NOP-Ins you will allow.
Please note that it is legitime for the Target to ignore
a NOP if the load is very high as those packet are mark
as IMMEDIATE.

Signed-off-by: Peter Lieven <pl@kamp.de>

lib/libiscsi.def

@@ -14,6 +14,7 @@ iscsi_get_fd
 iscsi_get_lba_status_sync
 iscsi_get_lba_status_task
 iscsi_get_target_address
+iscsi_get_nops_in_flight
 iscsi_inquiry_sync
 iscsi_inquiry_task
 iscsi_is_logged_in