Return -1 instead of calling pfatal in nmap_raw_socket.

There was one case where we previously didn't fatal, in nse_dnet.cc.
Move the fatal calls out of nmap_raw_socket and into the calling scope,
with the exception of the one in nse_dnet.cc.

The problem was reported by Rob Nicholls.
http://seclists.org/nmap-dev/2012/q4/186

osscan2.cc

@@ -1268,6 +1268,8 @@ HostOsScan::HostOsScan(Target *t) {
     rawsd = -1;
   } else {
     rawsd = nmap_raw_socket(t->deviceName());
+    if (rawsd < 0)
+      pfatal("socket troubles in %s", __func__);
     unblock_socket(rawsd);
     ethsd = NULL;
   }