ssh: enforce strict limits on DSA key parameters

The parseDSA function previously accepted DSA keys with arbitrary values
for the sub-prime Q and did not validate that group elements G and Y
were within the modulus P.

Malicious actors could provide a key with a massively large Q (e.g.,
millions of bits), leading to excessive CPU consumption during signature
verification.

This change restricts the sub-prime Q to exactly 160 bits, as required
by FIPS 186-2, and ensures that G and Y are strictly less than P.

This issue was found during a security audit by NCC Group Cryptography
Services, sponsored by Teleport.

Fixes golang/go#79565
Fixes CVE-2026-39829

Change-Id: I526118d94684076088d0625178844f64c1303ec8
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/781661
Reviewed-by: Roland Shoemaker <roland@golang.org>
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Neal Patel <nealpatel@google.com>