crypto: Enable SHA256 fingerprint checking in --verify-hash

crypto: Enable SHA256 fingerprint checking in --verify-hash

This enhances --verify-hash with an optional algorithm flag.  If not
provided, it defaults to SHA1 to preserve backwards compatbilitity with
existing configurations.  The only valid flags are SHA1 and SHA256.

In addition enhance the layout of the --verify-hash section in the man
page.

Signed-off-by: David Sommerseth <davids@openvpn.net>
Acked-by: Steffan Karger <steffan.karger@fox-it.com>
Message-Id: <20170504204201.1257-1-davids@openvpn.net>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg14538.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit 2193d7c08484d56ed07ba2e649abc2d08adcb245)