ssh/agent: enforce strict limits on DSA key parameters

The parseDSAKey function constructed a *dsa.PrivateKey directly from the
add-identity request without validating the key parameters. Unlike DSA
certificates, whose parameters are checked by the ssh package when the
certificate's public key is parsed, raw DSA keys added to the agent were
not validated at all.

Align the raw DSA key parsing with the validation already performed by
the main ssh package.

Fixes golang/go#79725

Change-Id: I537cc2175d35c19848c90c68739cf94ba7b50e10
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/795422
Reviewed-by: Roland Shoemaker <roland@golang.org>
Reviewed-by: Junyang Shao <shaojunyang@google.com>
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>