Virtual selectors

Where ordinary selectors specify a value stored in some object,
virtual selectors specify a function to call to compute a value
associated with some object.

Use them to replace the special case xdump ver by new table
EF_VERSION.

Move configkeys[] to lib/common because nsc_init() needs it to
initialize empfile[EF_VERSION].cadef.

src/lib/global/constants.c

@@ -221,3 +221,11 @@ float start_education = 0.0;
 float start_happiness = 0.0;
 float start_technology = 0.0;
 float start_research = 0.0;
+
+/* econfig keys */
+static int emp_config_dummy;
+struct keymatch configkeys[] = {
+#define	EMP_CONFIG_C_OUTPUT
+#include "econfig-spec.h"
+#undef	EMP_CONFIG_C_OUTPUT
+};