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/File.h

@@ -22,10 +22,10 @@ class RotateTimer;
 
 class BroFile : public BroObj {
 public:
-	BroFile(FILE* arg_f);
+	explicit BroFile(FILE* arg_f);
 	BroFile(FILE* arg_f, const char* filename, const char* access);
 	BroFile(const char* filename, const char* access, BroType* arg_t = 0);
-	virtual ~BroFile();
+	~BroFile() override;
 
 	const char* Name() const;