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.

src/lib/commands/xdump.c

@@ -112,7 +112,6 @@ xdprval(struct valstr *val, char *sep)
     unsigned char *s, *e, *l;
 
     switch (val->val_type) {
-    case NSC_TYPEID:
     case NSC_LONG:
 	pr("%s%ld", sep, val->val_as.lng);
 	break;