ssh: reject unsupported DSA key sizes
ssh: reject unsupported DSA key sizes

Fixes golang/go#19424.

Change-Id: I73370603dd612979420d608b73d67e673a52362b
Reviewed-on: https://go-review.googlesource.com/62870
Run-TryBot: Han-Wen Nienhuys <3269a793d567baa2fa5b041e3e5e0a5039790a2f@google.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Filippo Valsorda <c22b5f9178342609428d6f51b2c5af4c0bde6a42@filippo.io>
Reviewed-by: Adam Langley <db68fba232b2c49ed3e9d3081df349dde7b61dfd@golang.org>

golang/crypto,golang/crypto,golang/crypto,ScriptRock/crypto,ScriptRock/crypto