Introduce OpenSSH-compatible SHA256 key fingerprinting.

There's a new enumeration of fingerprint types, and you tell
ssh2_fingerprint() or ssh2_fingerprint_blob() which of them to use.

So far, this is only implemented behind the scenes, and exposed for
testcrypt to test. All the call sites of ssh2_fingerprint pass a fixed
default fptype, which is still set to the old MD5. That will change
shortly.

test/testcrypt.py

@@ -178,7 +178,7 @@ def make_argword(arg, argtype, fnname, argindex, to_preserve):
     if typename in {
             "hashalg", "macalg", "keyalg", "cipheralg",
             "dh_group", "ecdh_alg", "rsaorder", "primegenpolicy",
-            "argon2flavour"}:
+            "argon2flavour", "fptype"}:
         arg = coerce_to_bytes(arg)
         if isinstance(arg, bytes) and b" " not in arg:
             return arg