ssh: bound DH public values to [2, p-2].

Previously this code bounded the values to [1, p-1]. This protects
against invalid values that could take lots of CPU time to calculate
with. But the standard bounding is [2, p-2] so mirror that.

Since the DH exchange is signed anyway, this is not a security fix.

Change-Id: Ibef01805a596a433b0699d7a09c076344fa8c070
Reviewed-on: https://go-review.googlesource.com/30590
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>