Report security level instead of key size for 3DES

[phibos]

Reported by @tyll for pysslscan DinoTools/pysslscan#28 :

"3DES is currently reported with 168 bits because of its key size, but the security level is only 112 bits: https://www.ietf.org/rfc/rfc2246.txt (page 57) - IMHO it would be better to report the security level."

I think this information should be in the tls cipher suite registry.