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/ID.h

@@ -19,7 +19,7 @@ typedef enum { SCOPE_FUNCTION, SCOPE_MODULE, SCOPE_GLOBAL } IDScope;
 class ID : public BroObj {
 public:
 	ID(const char* name, IDScope arg_scope, bool arg_is_export);
-	~ID();
+	~ID() override;
 
 	const char* Name() const	{ return name; }