Add support for GnuPG extensions in private subkeys

[quinot]

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 

[kazu-yamamoto]

Sorry for the delay. I have merged your pull request.

[kazu-yamamoto]

v0.29 has been released. http://www.mew.org/~kazu/proj/pgpdump/en/