acpi_ibm: add support for ThinkPad PrivacyGuard

ThinkPad PrivacyGuard is a built-in toggleable privacy filter that
restricts viewing angles when on. It is an available on some new
ThinkPad models such as the X1 Carbon 7th gen (as an optional HW
upgrade).

The privacy filter can be enabled/disabled via an ACPI call. This commit
adds a sysctl under dev.acpi_ibm that allows for getting and setting the
PrivacyGuard state.

Submitted by: Kamila Součková <kamila@ksp.sk>
Reviewed By: cem, philip
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D23370