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/Val.cc

@@ -914,6 +914,11 @@ AddrVal::AddrVal(const char* text) : Val(TYPE_ADDR)
 	val.addr_val = new IPAddr(text);
 	}
 
+AddrVal::AddrVal(const std::string& text) : Val(TYPE_ADDR)
+	{
+	val.addr_val = new IPAddr(text);
+	}
+
 AddrVal::AddrVal(uint32 addr) : Val(TYPE_ADDR)
 	{
 	// ### perhaps do gethostbyaddr here?