Add support for GnuPG extensions in private subkeys

This change adds detection of GnuPG extensions in private/experimental
S2K specifiers (type 101), and parsing of gnu-dummy (1001) (indicating
absent secret key material), and gnu-divert-to-card (2002) (indicating
key material stored on a smartcard).

Example output
============

Keyring with primary secret key removed
----------------------------------------------------

    Old: Secret Key Packet(tag 5)(533 bytes)
            Ver 4 - new
            Public key creation time - Thu May 15 01:26:06 CEST 2014
            Pub alg - RSA Encrypt or Sign(pub 1)
            RSA n(4096 bits) - ...
            RSA e(17 bits) - ...
            Sym alg - CAST5(sym 3)
            GnuPG gnu-dummy (s2k 1001)

Secret subkey transferred to smartcard
--------------------------------------------------

    Old: Secret Subkey Packet(tag 7)(550 bytes)
            Ver 4 - new
            Public key creation time - Thu May 15 01:26:26 CEST 2014
            Pub alg - RSA Encrypt or Sign(pub 1)
            RSA n(4096 bits) - ...
            RSA e(17 bits) - ...
            Sym alg - CAST5(sym 3)
            GnuPG gnu-divert-to-card (s2k 1002)
            Serial Number: d2 76 00 01 24 01 02 00 00 05 00 00 1e 6d 00 00