Signed/unsigned warnings of MSVC resolved

Signed/unsigned warnings of MSVC resolved

This patch fixes the signed/unsigned comparison warnings discovered when
compiling openvpnserv using MSVC.

Wherever possible, it changes iterator and/or size variables to a more
appropriate type, or uses type-casting when it is safe to do so.
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20180413124756.5756-1-simon@rozman.si>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg16756.html

Signed-off-by: Gert Doering <gert@greenie.muc.de>