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/Base64.h

@@ -18,7 +18,7 @@ public:
 	// encode_base64()), encoding-errors will go to Reporter instead of
 	// Weird. Usage errors go to Reporter in any case. Empty alphabet
 	// indicates the default base64 alphabet.
-	Base64Converter(Connection* conn, const string& alphabet = "");
+	explicit Base64Converter(Connection* conn, const string& alphabet = "");
 	~Base64Converter();
 
 	// A note on Decode():