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/IPAddr.h

@@ -88,7 +88,7 @@ public:
 	 * @param s String containing an IP address as either a dotted IPv4
 	 * address or a hex IPv6 address.
 	 */
-	IPAddr(const BroString& s)
+	explicit IPAddr(const BroString& s)
 		{
 		Init(s.CheckString());
 		}