reintroduce md5_digest wrapper struct to fix gcc warnings

reintroduce md5_digest wrapper struct to fix gcc warnings

I was wrong to assume that adding the const qualifier to the pointer-to-
fixed-size-array contruction used in options_hash_changed_or_zero() was
allowed.  GCC actually warns about this, but I was using clang and clang
seems to be fine with the contruction.  To make GCC happy too, reintroduce
the md5_digest wrapped struct, and use that when passing around the digest.

This reverts the "struct md5_digest" parts of 827de237860813d2859a, but
keeps the rest.

Signed-off-by: Steffan Karger <steffan@karger.me>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <1437910039-30101-1-git-send-email-steffan@karger.me>
URL: http://article.gmane.org/gmane.network.openvpn.devel/9949
Signed-off-by: Gert Doering <gert@greenie.muc.de>