make tls-auth and tls-crypt per-connection-block options

make tls-auth and tls-crypt per-connection-block options

Different VPN servers may use different tls-auth/crypt keys.
For this reason it is convenient to make tls-auth/crypt
per-connection-block options so that the user is allowed to
specify one key per remote.

If no tls-auth/crypt option is specified in a given connection
block, the global settings, if any, are used.

Trac: #720
Cc: Steffan Karger <steffan@karger.me>
Signed-off-by: Antonio Quartulli <a@unstable.cc>
Acked-by: Steffan Karger <steffan.karger@fox-it.com>
Message-Id: <20180707090421.25953-2-a@unstable.cc>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg17226.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>