Mark one-parameter constructors as explicit & use override where possible

This commit marks (hopefully) ever one-parameter constructor as explicit.

It also uses override in (hopefully) all circumstances where a virtual
method is overridden.

There are a very few other minor changes - most of them were necessary
to get everything to compile (like one additional constructor). In one
case I changed an implicit operation to an explicit string conversion -
I think the automatically chosen conversion was much more convoluted.

This took longer than I want to admit but not as long as I feared :)

src/DNS_Mgr.cc

@@ -112,7 +112,7 @@ public:
 	IPAddr ReqAddr() const		{ return req_addr; }
 	string ReqStr() const
 		{
-		return req_host ? req_host : req_addr;
+		return req_host ? req_host : req_addr.AsString();
 		}
 
 	ListVal* Addrs();