Windows.reg: UAC yes/no

Password asking always would be more secure, but it's too annoying as
Windows doesn't have any kind of time when tasks can be performed
without password.

Also rename Windows.md to Windows.reg.markdown, it annoyed tabbing.
add IPv6.reg

IPv6.reg contains only Teredo configuration from Windows.reg. There is
also IPv6.reg.markdown to explain what it does.