Merge the ssh1_cipher type into ssh2_cipher.

The aim of this reorganisation is to make it easier to test all the
ciphers in PuTTY in a uniform way. It was inconvenient that there were
two separate vtable systems for the ciphers used in SSH-1 and SSH-2
with different functionality.

Now there's only one type, called ssh_cipher. But really it's the old
ssh2_cipher, just renamed: I haven't made any changes to the API on
the SSH-2 side. Instead, I've removed ssh1_cipher completely, and
adapted the SSH-1 BPP to use the SSH-2 style API.

(The relevant differences are that ssh1_cipher encapsulated both the
sending and receiving directions in one object - so now ssh1bpp has to
make a separate cipher instance per direction - and that ssh1_cipher
automatically initialised the IV to all zeroes, which ssh1bpp now has
to do by hand.)

The previous ssh1_cipher vtable for single-DES has been removed
completely, because when converted into the new API it became
identical to the SSH-2 single-DES vtable; so now there's just one
vtable for DES-CBC which works in both protocols. The other two SSH-1
ciphers each had to stay separate, because 3DES is completely
different between SSH-1 and SSH-2 (three layers of CBC structure
versus one), and Blowfish varies in endianness and key length between
the two.

(Actually, while I'm here, I've only just noticed that the SSH-1
Blowfish cipher mis-describes itself in log messages as Blowfish-128.
In fact it passes the whole of the input key buffer, which has length
SSH1_SESSION_KEY_LENGTH == 32 bytes == 256 bits. So it's actually
Blowfish-256, and has been all along!)

test/testcrypt.py

@@ -129,7 +129,7 @@ def make_argword(arg, argtype, fnname, argindex, to_preserve):
     if typename == "uint" and isinstance(arg, numbers.Integral):
         return "0x{:x}".format(arg)
     if typename in {
-            "hashalg", "macalg", "keyalg", "ssh1_cipheralg", "ssh2_cipheralg",
+            "hashalg", "macalg", "keyalg", "cipheralg",
             "dh_group", "ecdh_alg", "rsaorder"}:
         arg = unicode_to_bytes(arg)
         if isinstance(arg, bytes) and b" " not in arg: