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/analyzer/protocol/tcp/TCP_Reassembler.h

@@ -22,7 +22,7 @@ public:
 	TCP_Reassembler(Analyzer* arg_dst_analyzer, TCP_Analyzer* arg_tcp_analyzer,
 	                Type arg_type, TCP_Endpoint* arg_endp);
 
-	virtual ~TCP_Reassembler();
+	~TCP_Reassembler() override;
 
 	void Done();