tls-crypt: introduce tls_crypt_kt()

tls-crypt: introduce tls_crypt_kt()

Reduces code duplication (and prepares for tls-crypt-v2, which needs the
same functionality at more places).

Because tls_crypt_kt() is a static function we now need to include
tls_crypt.c from the tests, rather than tls_crypt.h.

Signed-off-by: Steffan Karger <steffan.karger@fox-it.com>
Acked-by: Antonio Quartulli <a@unstable.cc>
Message-Id: <1502531632-16833-1-git-send-email-steffan.karger@fox-it.com>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg15229.html
Signed-off-by: David Sommerseth <davids@openvpn.net>
(cherry picked from commit 489c7bf93ec618e03dbd9618efbb6e251a65e76c)