Add support for cracking very old GPG keys

Links,

• http://www.openwall.com/lists/john-users/2018/06/18/8 (has data)

• http://www.openwall.com/lists/john-users/2018/06/19/2

Quick notes,

On CentOS 7,

$ gpg --homedir . --s2k-cipher-algo idea --s2k-mode 0 --simple-sk-checksum --gen-key

$ pgpdump secring.gpg
...
Old: Secret Key Packet(tag 5)(931 bytes)
	Ver 4 - new
	Public key creation time - Tue Jun 19 15:04:57 IST 2018
	Pub alg - RSA Encrypt or Sign(pub 1)
	RSA n(2048 bits) - ...
	RSA e(17 bits) - ...
	Sym alg - IDEA(sym 1)
	Simple string-to-key(s2k 0):
		Hash alg - SHA1(hash 2


$ pgpdump newold.asc
Old: Secret Key Packet(tag 5)(928 bytes)
	Ver 3 - old
	Public key creation time - Tue Jun 19 00:59:57 IST 2018
	Valid days - 0[0 is forever]
	Pub alg - RSA Encrypt or Sign(pub 1)
	RSA n(2048 bits) - ...
	RSA e(5 bits) - ...
	Sym alg - IDEA(sym 1)
	Simple string-to-key for IDEA
	IV - 4c 24 29 7b 7d fe f2 3d
	Encrypted RSA d(2046 bits) - ...
	Encrypted RSA p(1024 bits) - ...
	Encrypted RSA q(1024 bits) - ...
	Encrypted RSA u(1022 bits) - ...
	Checksum - 52 27


It seems that we don't support this (i.e. Simple string-to-key for IDEA) S2K yet.

Note: Rejecting such keys and avoiding false negatives would be helpful for now.

CC @solardiz.