Replace OS_SPECIFIC_DIRSEP with PATH_SEPARATOR

Replace OS_SPECIFIC_DIRSEP with PATH_SEPARATOR

We have two define that do exactly the same. Also move the check
from configure.ac to syshead.h since it is really only checking
for Windows.

Patch V2: Also remove from config-msvc.h

Signed-off-by: Arne Schwabe <arne@rfc2549.org>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20210422152939.2134046-5-arne@rfc2549.org>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg22203.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>