Symbolic names for table entries in conditionals, closes #928376:

Symbolic names for table entries in conditionals, closes #928376:
(ef_elt_byname): New.
(nstr_match_val): Use it to generalize to arbitrary table with
uniquely named elements.  Use ca_type to find table, remove parameter
type.  Callers changed.
(nstr_mkselval): Adapt sanity check accordingly.
(NSC_TYPEID): No longer needed, remove, users changed to use
appropriate integer type instead.  This fixes signedness of sector
selectors des and newdes.
(meta_type): Remove entry for NSC_TYPEID.
(nstr_coerce_val): Can't detect typeid - integer mismatch anymore.
Was nice to have; perhaps revive it later.