Fix some more wrong defines in config-msvc.h

Fix some more wrong defines in config-msvc.h

Not sure where these came from, but here goes:

S_IRUSR = 0 --> _S_IREAD
S_IWUSR = 0 --> _S_IWRITE

ENABLE_DEBUG is on, but I do not think we want it in production build
(removed).

S_IRGRP is not defined but seems to be used. I have added it, remove if
not required.
This define is based on mingw and matches MS docs on <filesystem>
(https://docs.microsoft.com/en-us/cpp/standard-library/filesystem-enumerati
ons?view=msvc-160)

Trac: #1430
Acked-by: Lev Stipakov <lstipakov@gmail.com>
Message-Id: <20211015175330.15760-1-selva.nair@gmail.com>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg22942.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>