The NSC_CONST flags added in commit fa63f87b have always been missing
here. The previous commit changed xdump meta meta. Other than that,
it's just different encodings.
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.
