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_analysis/analyzer/x509/X509Common.h

@@ -16,7 +16,7 @@ namespace file_analysis {
 
 class X509Common : public file_analysis::Analyzer {
 public:
-	virtual ~X509Common() {};
+	~X509Common() override {};
 
 	/**
 	 * Retrieve an X509 extension value from an OpenSSL BIO to which it was