Rename OPT_P_IPWIN32 to OPT_P_DHCPDNS and include --dns in it

Rename OPT_P_IPWIN32 to OPT_P_DHCPDNS and include --dns in it

The dns options are very similar to dhcp-option and should fall
under the same option mask. For that rename the OPT_P_IPWIN32 mask
to OPT_P_DHCPDNS and include dns in it.

This effects currently route-nopull which block all host side
network/dns configuration but did not block the new dns option.
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20220817075925.815184-1-arne@rfc2549.org>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg24946.html

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