Clarify that DH parameters are unsigned

[pasi.eronen@…]

​http://www1.ietf.org/mail-archive/web/tls/current/msg01101.html

On a related note, my large integer code prepends a leading
zero byte if the most significant bit is a one to prevent it
from appearing to be negative.  So when transferring the
Diffie-Hellman p, g, Ys, and Yc, they might have a leading
zero byte.  I haven't noticed any problems with this, but
wondered if this should be clarified to either always do it,
never do it, or leave it up to the implementer.

​http://www1.ietf.org/mail-archive/web/tls/current/msg01105.html

Good catch! Perhaps we should add something like this to
Section 7.4.3?

   "The modulus (and other Diffie-Hellman parameters) are
   sent as unsigned integers (i.e., leading zero octets are
   not required even if the most significant bit is set)."