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.

include/file.h

@@ -142,6 +142,7 @@ enum {
     EF_INFRASTRUCTURE,
     EF_UPDATES,			/* not actually static */
     EF_TABLE,
+    EF_VERSION,
     EF_META,			/* not really configuration */
     /* Symbol tables */
     EF_AGREEMENT_STATUS,