Allocate the exclusion set before adding client-supplied usernames. A true Exclude entry otherwise panics when SSH is enabled.
Exercise the c2n handler with true, false, absent, and unknown exclusions, and cover the result cap and disabled-SSH behavior.
Updates tailscale/corp#50380
Signed-off-by: James Tucker <james@tailscale.com>
Change-Id: I1d9faf36e5809ccff888fc402329652e8b7d88c8