Updates tailscale/corp#25936
This defines a new syspolicy 'Hostname' and allows an IT administrator to override the value we normally read from os.Hostname(). This is particularly useful on Android and iOS devices, where the hostname we get from the OS is really just the device model (a platform restriction to prevent fingerprinting).
If we don't implement this, all devices on the customer's side will look like `google-pixel-7a-1`, `google-pixel-7a-2`, `google-pixel-7a-3`, etc. and it is not feasible for the customer to use the API or worse the admin console to manually fix these names.
Apply code review comment by @nickkhyl
Signed-off-by: Andrea Gottardo <andrea@gottardo.me>
Co-authored-by: Nick Khyl <1761190+nickkhyl@users.noreply.github.com>
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