control/controlclient: tell control when Ubuntu has disabled Tailscale's sources

[bradfitz]

(Related but separate from #2500)

I just upgraded my Ubuntu desktop to Impish, which again disabled my /etc/apt/sources.list.d/tailscale.list file, commenting it out with:

# disabled on upgrade to impish

... at the end.

#2500 tracks fighting that and undoing it.

But this bug tracks a safer thing we can always do: have tailscaled detect that it's commented out and report that to control, so we can surface it as a warning in the admin panel.

/cc @danderson @DentonGentry @crawshaw