Remove a number of platform specific checks in configure.ac

Remove a number of platform specific checks in configure.ac

- Remove windows XP/old mingw compat code in socket.c
- Use _WIN32 instead checking for existence of windows.h, winsock2.h
  and ws2tcpip.h in autconf
- Remove check for unlink. The last remaining use is a check inside
  a Unix socket.
- Even Windows has umask, so remove the check for it
- Move epoll.h inclusion to event.c

Patch V2: Add epoll.h syshead.h that accidently was put into another
          patch

Signed-off-by: Arne Schwabe <arne@rfc2549.org>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20210422152939.2134046-6-arne@rfc2549.org>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg22201.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>