Update cipher string and explain it with more detail

I don't think this makes a functional difference, but the intent
behind each component is now more clear.  These decisions were
informed by reading s2_lib.c and s3_lib.c.

The express banning of DES has been lifted, because per the standard
is too weak a cipher to be in 'HIGH'.  The forbidding of 3DES, which
can have long enough keys to be in the 'HIGH' category, is still
explicitly forbidden for performance reasons.

SSLv2 has been expressly banned for lucidity sake, even though it is
not strictly necessary given "!3DES": thanks to accidents of history
the only 'HIGH'-class cipher suite present is based on 3DES.  Thus
'HIGH:[...]:!3DES' should be sufficient to the same meaning, but only
through a close reading of the availability of what's in SSLv2.

Also deny use of cipher suites using MD5.  It's not clear this can
mount a practical attack against a SSLv3 session, and seems even less
likely so in TLS v1.0+ since they use HMAC, where MD5 is thought to be
fine, but given that flavors of SHA are widely deployed and thought to
be stronger, it doesn't seem such a ban will be of predicable
practical consequence to compatibility.

At the time of writing, the cipher suites that OpenSSL will support
given these constraints are:

$ openssl version
OpenSSL 1.0.1c 10 May 2012

(Line breaks inserted for clarity below)

$ openssl ciphers 'HIGH:!aNULL:!SSLv2:!RC4:!3DES:!MD5'
ECDHE-RSA-AES256-GCM-SHA384:
ECDHE-ECDSA-AES256-GCM-SHA384:
ECDHE-RSA-AES256-SHA384:
ECDHE-ECDSA-AES256-SHA384:
ECDHE-RSA-AES256-SHA:
ECDHE-ECDSA-AES256-SHA:
SRP-DSS-AES-256-CBC-SHA:
SRP-RSA-AES-256-CBC-SHA:
DHE-DSS-AES256-GCM-SHA384:
DHE-RSA-AES256-GCM-SHA384:
DHE-RSA-AES256-SHA256:
DHE-DSS-AES256-SHA256:
DHE-RSA-AES256-SHA:
DHE-DSS-AES256-SHA:
DHE-RSA-CAMELLIA256-SHA:
DHE-DSS-CAMELLIA256-SHA:
ECDH-RSA-AES256-GCM-SHA384:
ECDH-ECDSA-AES256-GCM-SHA384:
ECDH-RSA-AES256-SHA384:
ECDH-ECDSA-AES256-SHA384:
ECDH-RSA-AES256-SHA:
ECDH-ECDSA-AES256-SHA:
AES256-GCM-SHA384:
AES256-SHA256:
AES256-SHA:
CAMELLIA256-SHA:
PSK-AES256-CBC-SHA:
ECDHE-RSA-AES128-GCM-SHA256:
ECDHE-ECDSA-AES128-GCM-SHA256:
ECDHE-RSA-AES128-SHA256:
ECDHE-ECDSA-AES128-SHA256:
ECDHE-RSA-AES128-SHA:
ECDHE-ECDSA-AES128-SHA:
SRP-DSS-AES-128-CBC-SHA:
SRP-RSA-AES-128-CBC-SHA:
DHE-DSS-AES128-GCM-SHA256:
DHE-RSA-AES128-GCM-SHA256:
DHE-RSA-AES128-SHA256:
DHE-DSS-AES128-SHA256:
DHE-RSA-AES128-SHA:
DHE-DSS-AES128-SHA:
DHE-RSA-CAMELLIA128-SHA:
DHE-DSS-CAMELLIA128-SHA:
ECDH-RSA-AES128-GCM-SHA256:
ECDH-ECDSA-AES128-GCM-SHA256:
ECDH-RSA-AES128-SHA256:
ECDH-ECDSA-AES128-SHA256:
ECDH-RSA-AES128-SHA:
ECDH-ECDSA-AES128-SHA:
AES128-GCM-SHA256:
AES128-SHA256:
AES128-SHA:
CAMELLIA128-SHA:
PSK-AES128-CBC-SHA