Validate DNS domain name before powershell invocation

Validate DNS domain name before powershell invocation

Starting from commit

  d383d6e ("win: replace wmic invocation with powershell")

we pass --dhcp-option DOMAIN value to a powershell command
to set DNS domain. Without validation this opens the door
to a command injection atack.

This only allows domain names with characters:

  [A-Za-z0-9.-_\x80-\0xff]

Change-Id: I7a57d7b4e84aa2b9c9e71e30520ed468b0e3c278
Signed-off-by: Lev Stipakov <lev@openvpn.net>
Acked-by: Gert Doering <gert@greenie.muc.de>
Gerrit URL: https://gerrit.openvpn.net/c/openvpn/+/1198
Message-Id: <20250918173447.32466-1-gert@greenie.muc.de>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg33071.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>