Refactor static/tls-auth key loading

Refactor static/tls-auth key loading

Remove duplicate code, in preparation for adding --tls-crypt, which
otherwise would have to duplicate this code again.

This should be equivalent to the old code, except for two things:
* The log lines for static key initialization change slightly, from
  "Static Encrypt/Decrypt" to "Incoming/Outgoing Static Key Encryption"
* We also 'check and fix highly unlikely key problems' for tls-auth
  keys (boils down to a sanity-check for an all-zero key).

Signed-off-by: Steffan Karger <steffan.karger@fox-it.com>
Acked-by: Arne Schwabe <arne@rfc2549.org>
Message-Id: <1478636302-9678-2-git-send-email-steffan.karger@fox-it.com>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg12969.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>