In this PR, we move the code that checks the AlwaysOn policy from ipnserver.actor to ipnauth.
It is intended to be used by ipnauth.Actor implementations, and we temporarily make it exported
while these implementations reside in ipnserver and in corp. We'll unexport it later.
We also update [ipnauth.Actor.CheckProfileAccess] to accept an auditLogger, which is called
to write details about the action to the audit log when required by the policy, and update
LocalBackend.EditPrefsAs to use an auditLogger that writes to the regular backend log.
Updates tailscale/corp#26146
Signed-off-by: Nick Khyl <nickk@tailscale.com>
Nick Khyl