[iSCSI] Support Windows Server 2008 direct iSCSI installation

Add yet another ugly hack to iscsiboot.c, this time to allow the user to
inhibit the shutdown/removal of the iSCSI INT13 device (and the network
devices, since they are required for the iSCSI device to function).

On the plus side, the fact that shutdown() now takes flags to
differentiate between shutdown-for-exit and shutdown-for-boot means that
another ugly hack (to allow returning via the PXE stack on BIOSes that
have broken INT 18 calls) will be easier.

I feel dirty.

src/usr/autoboot.c

@@ -41,6 +41,9 @@
 /** Time to wait for link-up */
 #define LINK_WAIT_MS 15000
 
+/** Shutdown flags for exit */
+int shutdown_exit_flags = 0;
+
 /**
  * Identify the boot network device
  *