gpg: Print all keys with --decrypt --list-only.

* g10/mainproc.c (proc_pubkey_enc): Use dedicated error code for
list-only and put the key into PKENC_LIST.
(print_pkenc_list): Take care of the new error code.

If the secret keys exist in --list-only mode it was not printed in
--list-only mode.

• GnuPG-bug-id: T3718
• Signed-off-by: Werner Koch <wk@gnupg.org>