asn1: Add --decorate=... for internal bookkeeping

This option, `--decorate=TYPE-NAME:FIELD-TYPE:field-name[?]` allows one to add
a field to any struct generated by the ASN.1 compiler for any SET or SEQUENCE
type such that:

 - the field will     be freed by the `free_TYPE_NAME()` function
 - the field will     be copied by the `copy_TYPE_NAME()` function
 - the field will not be printed by the `print_TYPE_NAME()` function
 - the field will NOT be encoded or decoded

This is useful for internal bookkeeping.

The first use of this may well be for adding an optional field to
`Principal` where information about name attributes will be stored,
which will then allow us to have GSS name attributes for the krb5
mechanism w/o having to refactor the mechanism to use a different
structure for representing `gss_name_t` mechnames than the one currently
used (`Principal`; `krb5_principal` happens to be a typedef alias of
`Principal *`).

So w/o massive rototilling of the GSS krb5 mechanism we can have name
attributes, _and_ we'll also be able to have those in the krb5 API as
well w/o any massive rototilling there either.

lib/asn1/gen.c

@@ -1355,6 +1355,8 @@ define_type(int level, const char *name, const char *basename, Type *pt, Type *t
     case TSet:
     case TSequence: {
 	Member *m;
+        char *ft, *fn;
+        int deco_opt;
 
 	getnewbasename(&newbasename, typedefp || level == 0, basename, name);
 
@@ -1395,6 +1397,13 @@ define_type(int level, const char *name, const char *basename, Type *pt, Type *t
             fprintf(jsonfile, ",\"opentype\":");
             define_open_type(level, newbasename, name, basename, t, t);
         }
+        if (decorate_type(newbasename, &ft, &fn, &deco_opt)) {
+	    space(level + 1);
+            fprintf(headerfile, "%s %s%s;\n", ft, deco_opt ? "*" : "", fn);
+            fprintf(jsonfile, ",\"decorate\":{\"type\":\"%s\",\"name\":\"%s\", \"optional\":%s}", ft, fn, deco_opt ? "true" : "false");
+            free(ft);
+            free(fn);
+        }
 	space(level);
 	fprintf (headerfile, "} %s;\n", name);
 	break;