updates tailscale/corp#26435
Adds client support for sending audit logs to control via /machine/audit-log.
Specifically implements audit logging for user initiated disconnections.
This will require further work to optimize the peristant storage and exclusion
via build tags for mobile:
tailscale/corp#27011tailscale/corp#27012
Signed-off-by: Jonathan Nobels <jonathan@tailscale.com>