gpg: New option --list-filter

* g10/gpg.c (oListFilter): New.
(opts): Add --list-filter.
(main): Parse oListFilter.
* g10/keylist.c: Include init.h and recsel.h.
(struct list_filter_s, list_filter): New.
(release_list_filter): New.
(cleanup_keylist_globals): New.
(parse_and_set_list_filter): New.
(list_keyblock): Implement --list-filter type "select".

* g10/import.c (impex_filter_getval): Add scope support and new
property names "key-size", "algostr", "origin", "lastupd", and "url".
--

This option is pretty useful to select keys based on their properties.
The scope thing can be sued to limit a selection to just the primary
key or to subkeys.  For example:

  gpg -k --list-filter 'select=revoked-f && sub/algostr=ed25519'

Lists all non-revoked keys with an ed25519 (signing)-subkey.

g10/test-stubs.c

@@ -572,3 +572,11 @@ get_revocation_reason (PKT_signature *sig, char **r_reason,
     *r_comment = NULL;
   return 0;
 }
+
+const char *
+impex_filter_getval (void *cookie, const char *propname)
+{
+  (void)cookie;
+  (void)propname;
+  return NULL;
+}