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/common/xundump.c

@@ -447,7 +447,6 @@ setnum(int fldno, double dbl)
 
     switch (ca->ca_type) {
     case NSC_CHAR:
-    case NSC_TYPEID:
 	old = ((signed char *)memb_ptr)[idx];
 	((signed char *)memb_ptr)[idx] = (signed char)dbl;
 	break;