rework sslv2 handling

[tintinweb]

• fix dissector https://stackoverflow.com/questions/36622575/scapy-send-sniff-packet-with-sslv2record-not-parsed-correctly
• add sslv2 cipher RC2_CBC_128_MD5
• always perform i2m adjustment if set
• add SSLv2 unittests

[tintinweb]

unittests are happy; manually verified SSLv2, this is good to merge.

>>> SSL(str(SSL(records=[SSLv2Record(length=0x591, content_type=5)/"\xbf"])))
<SSL  records=[<SSLv2Record  length=0x591 content_type=5 |<Raw  load='\xbf' |>>] |>```

[alexmgr]

Do you need to add RC2_CBC_128_CBC_WITH_MD5 to TLSSecurityParameters.crypto-params for future decryption to work?
Other then that, knock yourself out ;)

[tintinweb]

I do not think we need to put effort into adding it as it is't really widely used :)