Fixed a couple nsock problems described in

http://seclists.org/nmap-dev/2012/q3/56.  r29134 already addressed the issue but
was incomplete.

This replaces r29134 with an engine-agnostic approach, and additionally enforces
the reset of IOD flags before use or re-use.

nsock/src/nsock_iod.c

@@ -121,6 +121,8 @@ nsock_iod nsi_new2(nsock_pool nsockp, int sd, void *userdata) {
   nsi->userdata = userdata;
   nsi->nsp = (mspool *)nsockp;
 
+  nsi->_flags = 0;
+
   nsi->read_count = 0;
   nsi->write_count = 0;