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/logging/writers/sqlite/Plugin.cc

@@ -10,7 +10,7 @@ namespace Bro_SQLiteWriter {
 
 class Plugin : public plugin::Plugin {
 public:
-	plugin::Configuration Configure()
+	plugin::Configuration Configure() override
 		{
 		AddComponent(new ::logging::Component("SQLite", ::logging::writer::SQLite::Instantiate));