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/dh.h b/dh.h
index dfc1480..63a1b14 100644
--- a/dh.h
+++ b/dh.h
@@ -1,4 +1,4 @@
-/* $OpenBSD: dh.h,v 1.10 2008/06/26 09:19:40 djm Exp $ */
+/* $OpenBSD: dh.h,v 1.12 2015/01/19 20:16:15 markus Exp $ */
 
 /*
  * Copyright (c) 2000 Niels Provos.  All rights reserved.
@@ -38,11 +38,12 @@
 DH	*dh_new_group1(void);
 DH	*dh_new_group14(void);
 
-void	 dh_gen_key(DH *, int);
+int	 dh_gen_key(DH *, int);
 int	 dh_pub_is_valid(DH *, BIGNUM *);
 
-int	 dh_estimate(int);
+u_int	 dh_estimate(int);
 
+/* Min and max values from RFC4419. */
 #define DH_GRP_MIN	1024
 #define DH_GRP_MAX	8192