external/openssh: update to 6.8p1.

In preparation for some updates to external/openssh to make it work with
BoringSSL, this change updates the code to a recent version. The current
version (5.9p1) is coming up on four years old now.

  * Confirmed that f5c67b478bef9992de9e9ec91ce10af4f6205e0d matches
    OpenSSH 5.9p1 exactly (save for the removal of the scard
    subdirectory).

  * Downloaded openssh-6.8p1.tar.gz (SHA256:
    3ff64ce73ee124480b5bf767b9830d7d3c03bbcb6abe716b78f0192c37ce160e)
    and verified with PGP signature. (I've verified Damien's key in
    person previously.)

  * Applied changes between f5c67b478bef9992de9e9ec91ce10af4f6205e0d and
    OpenSSH 5.9p1 to 6.8p1 and updated the build as best I can. The
    ugliest change is probably the duplication of umac.c to umac128.c
    because Android conditionally compiles that file twice. See the
    comment in those files.

Change-Id: I63cb07a8118afb5a377f116087a0882914cea486
diff --git a/clientloop.h b/clientloop.h
index a259b5e..338d451 100644
--- a/clientloop.h
+++ b/clientloop.h
@@ -1,4 +1,4 @@
-/* $OpenBSD: clientloop.h,v 1.28 2011/06/22 22:08:42 djm Exp $ */
+/* $OpenBSD: clientloop.h,v 1.31 2013/06/02 23:36:29 dtucker Exp $ */
 
 /*
  * Author: Tatu Ylonen <ylo@cs.hut.fi>
@@ -70,6 +70,7 @@
 #define SSHMUX_COMMAND_STDIO_FWD	4	/* Open stdio fwd (ssh -W) */
 #define SSHMUX_COMMAND_FORWARD		5	/* Forward only, no command */
 #define SSHMUX_COMMAND_STOP		6	/* Disable mux but not conn */
+#define SSHMUX_COMMAND_CANCEL_FWD	7	/* Cancel forwarding(s) */
 
 void	muxserver_listen(void);
 void	muxclient(const char *);