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/common/emp_config.c

@@ -55,16 +55,6 @@
 #include "optlist.h"
 #include "prototypes.h"
 
-/* Dummy one */
-static int emp_config_dummy;
-
-/* things that can be changed */
-struct keymatch configkeys[] = {
-#define	EMP_CONFIG_C_OUTPUT
-#include "econfig-spec.h"
-#undef	EMP_CONFIG_C_OUTPUT
-};
-
 static struct keymatch *keylookup(char *key, struct keymatch tbl[]);
 static int set_paths(char *);