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

FPEngine.cc

@@ -187,6 +187,8 @@ void FPNetworkControl::init(const char *ifname, devtype iftype) {
     if (this->rawsd >= 0)
       close(this->rawsd);
     rawsd = nmap_raw_socket(ifname);
+    if (rawsd < 0)
+      pfatal("Couldn't obtain raw socket in %s", __func__);
   }
 
   /* De-register existing callers */