Fix accidental change to connshare pipe naming.

Jacob spots that on Windows, current PuTTY is not compatible with
0.74, if one of them acts as a connection sharing upstream and the
other as a downstream. That's because commit 1344d4d1
accidentally changed the hash preimage in capi_obfuscate_string() so
that it no longer had an SSH-like string length field at the front. So
the two versions of PuTTY will expect the named pipe to have a
different pathname, and so they won't be able to find each other.

Interoperation between PuTTY versions is not the most important use
case of connection sharing - surely the typical user will invoke it by
activating the same session twice, or by using Duplicate Session. But
it was never intended to deliberately _not_ work, so let's fix it
before 0.75 goes out, so that at least the incompatible behaviour will
only ever have appeared in development snapshots.

windows/wincapi.c

@@ -71,7 +71,11 @@ char *capi_obfuscate_string(const char *realname)
      * We don't want to give away the length of the hostname either,
      * so having got it back out of CryptProtectMemory we now hash it.
      */
-    hash_simple(&ssh_sha256, make_ptrlen(cryptdata, cryptlen), digest);
+    {
+        ssh_hash *h = ssh_hash_new(&ssh_sha256);
+        put_string(h, cryptdata, cryptlen);
+        ssh_hash_final(h, digest);
+    }
 
     sfree(cryptdata);