nselib: Fix EdDSA in default TLS 1.2 signature_algorithms

[droe]

I seem to have lost my commit bit on the Subversion repo (which is fine, kudos for the user lifecycle management), so here's a fix I'd like to see in Nmap:

Fix the default signature_algorithms extension used for TLS 1.2 to include code points 0x0807 (intrinsic, ed25519) and 0x0808 (intrinsic, ed448) instead of the invalid 0x0407, 0x0408, 0x0607 and 0x0608, combining sha256/sha512 with EdDSA.

Quoting RFC 8422:

5.1.3. The signature_algorithms Extension and EdDSA

The signature_algorithms extension, defined in Section 7.4.1.4.1 of
[RFC5246], advertises the combinations of signature algorithm and
hash function that the client supports. The pure (non-prehashed)
forms of EdDSA do not hash the data before signing it. For this
reason, it does not make sense to combine them with a hash function
in the extension.

For bits-on-the-wire compatibility with TLS 1.3, we define a new
dummy value in the "TLS HashAlgorithm" registry that we call
"Intrinsic" (value 8), meaning that hashing is intrinsic to the
signature algorithm.
To represent ed25519 and ed448 in the signature_algorithms extension,
the value shall be (8,7) and (8,8), respectively.