Guard double authenticate calls

Whilst authenticating we emit signals and expect callbacks to prompts.
The pam worker is inside pam_authenticate in the nested event loop.

It is not expected for GUI clients to call authenticate twice without a
cancel inbetween, but we should still guard against it as it causes a
2 jobs for !72 with work/double_auth_guard in 4 minutes and 11 seconds (queued for 9 seconds)
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