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/UID.h

@@ -28,7 +28,7 @@ public:
 	 * Construct a UID of a given bit-length, optionally from given values.
 	 * @see UID::Set
 	 */
-	UID(bro_uint_t bits, const uint64* v = 0, size_t n = 0)
+	explicit UID(bro_uint_t bits, const uint64* v = 0, size_t n = 0)
 		{ Set(bits, v, n); }
 
 	/**