[patch 1/2] use chacha20 from openssl (1.1.0+) when possible

[Yuriy M. Kaminskiy]

On some cpu's optimized chacha implementation in openssl (1.1.0+) is
notably faster (and on others it is just faster) than generic C
implementation in openssh.

Sadly, openssl's chacha20-poly1305 (EVP_chacha20_poly1305) uses
different scheme (with padding/etc - see rfc8439) and it looks it is not
possible to use in openssh.

OpenSSL 1.1.1+ also exports "raw" poly1305 primitive, but I
have not tried it yet (it was not in 1.1.0).

Trivial benchmark:
time ssh -c chacha20-poly1305 at openssh.com -S none -o Compression=no \
localhost 'dd if=/dev/zero bs=100000 count=10000' >/dev/null
(comparing "user time" only)

openssh: 7.9p1, self-compiled, based on upstream package from 
debian/unstable, hostkey - ecdsa/p256, pubkey auth key - ecdh/p256

Machine: pretty old amd k8 (w/ SSE2, but no SSSE3/AVX/AESNI)
OS: linux/debian/stretch, openssl 1.1.0j-1deb9u1
i386: speed: +8%
amd64: speed: +10%

Machine: raspberry pi 3b+ (BCM2837B0, 4-core Cortex-A53 @1.4GHz)
OS: raspbian/stretch

baseline: armhf/raspbian: unpatched ssh-7.9p1: 30.8s

with openssl 1.1.0j-1deb9u1 from raspbian (compiled for armv6 without neon):

armhf/raspbian: 24.7 seconds, speed: +25%

with openssl 1.1.0j-1deb9u1 from debian/stretch/armhf (compiled for
armv7 with neon autodetection):
armhf: 22.2 seconds, speed: +39%

Patches against 7.9p1 (tested) and git master (untested, only resolved 
configure.ac conflict) attached.
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