devd: minor nits

- mark usage as noreturn
- config does not need a virtual destructor

sbin/devd/devd.cc

@@ -161,7 +161,7 @@ static const char *configfile = CF;
 static void devdlog(int priority, const char* message, ...)
 	__printflike(2, 3);
 static void event_loop(void);
-static void usage(void);
+static void usage(void) __dead2;
 
 template <class T> void
 delete_and_clear(vector<T *> &v)

sbin/devd/devd.hh

@@ -147,7 +147,7 @@ class config
 {
 public:
 	config() { push_var_table(); }
-	virtual ~config() { reset(); }
+	~config() { reset(); }
 	void add_attach(int, event_proc *);
 	void add_detach(int, event_proc *);
 	void add_directory(const char *);