control/controlclient,posture,util/syspolicy: use predefined syspolicy keys instead of string literals

With the upcoming syspolicy changes, it's imperative that all syspolicy keys are defined in the syspolicy package
for proper registration. Otherwise, the corresponding policy settings will not be read.

This updates a couple of places where we still use string literals rather than syspolicy consts.

Updates #12687

Signed-off-by: Nick Khyl <nickk@tailscale.com>
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