Enforce acceptable range for Diffie-Hellman server value.

Enforce acceptable range for Diffie-Hellman server value.

Florent Daigniere of Matta points out that RFC 4253 actually
_requires_ us to refuse to accept out-of-range values, though it isn't
completely clear to me why this should be a MUST on the receiving end.

Matta considers this to be a security vulnerability, on the grounds
that if a server should accidentally send an obviously useless value
such as 1 then we will fail to reject it and agree a key that an
eavesdropper could also figure out. Their id for this vulnerability is
MATTA-2015-002.