fw: a dead firewall leaves the network down, and can be restarted into it

Two things can happen when a firewall dies: the traffic it was
filtering carries on unfiltered, or it stops.  Only the second is
defensible.  A machine briefly off the network is a machine somebody
notices and fixes; a machine briefly on the network with no rules is
the thing the firewall was installed to prevent, and nobody notices it
at all.

fw already does the second, in all three modes, and by mechanism rather
than by care.  Measured rather than assumed:

	before fw   device: /net/ether1  addr: 10.9.9.1  route: 10.9.9.254
	fw running  device: pkt0         addr: 10.9.9.1  route: 10.9.9.254
	fw killed   device:              addr:           route:

pktmedium is unbindonclose, so the interface and the address go when
fw's fds close, and the card is left bound to nothing with nothing
reading it.  In a namespace it is harder still: /net answers "i/o on
hungup channel" and bind -a '#I' /net answers "mount/attach disallowed",
because the device mask was dropped before the program started.

So todo item 1 -- "a dead fw takes the network with it", open since the
first commit -- was the requirement written down as a defect.  It is
now design.md and a FAILURE section in fw(8), and the tests assert it,
which is the point: this is exactly the property a later helpful change
reverses without meaning to.  putback() was that change, written and
never run; deleting it removed a fail-open path, not just dead code.

What was actually broken is recovery, and in a way nobody had reached:
a fw that dies leaves its control filesystem mounted, and a corpse of a
mount fails everything asked of it -- including the access() check fw
makes before touching a card, which then refuses the restart:

	fw: /tmp/rdbg/ctl: clone failed; not touching /net/ether1 until it exists

That check exists so fw does not take a card it cannot then serve, and
it was keeping fw from ever coming back.  Now the dead mount is cleared
first, the same way reclaim() clears the pkt interface the same dead fw
left behind: both are its own wreckage.

With that and -a/-g in the service file -- so a restart does not need
the address it just lost -- the whole cycle works and never passes
through open: crash, network down, restart, network up and filtered.
Ten checks, three of which fail against the previous fw.c.  87 pass.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

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