DEBUG add function to set debug fd

DPRINTF was blindly sending all debug output to fd 2 (stderr).
The new function iscsi_set_debug_fd() will set the debug fd.
In general debugging is completely disabled by default.

lib/libiscsi.def

@@ -65,6 +65,7 @@ iscsi_scsi_task_cancel
 iscsi_service
 iscsi_set_alias
 iscsi_set_debug
+iscsi_set_debug_fd
 iscsi_set_header_digest
 iscsi_set_initiator_username_pwd
 iscsi_set_isid_en