fw: fix a bypass, a broken round-trip, and eleven others from review

The serious one is that the ctl filter was a blacklist.  checkctl looked
at connect and announce and passed everything else, but udpctl takes
"headers", and udpcreate gives a conversation a live write queue at clone
time:

	c->wq = qbypass(udpkick, c);

So three writes -- clone, "headers", a header-prefixed datagram to data --
sent a packet anywhere, with no connect for a rule to match.  rudp and
icmpv6 have the same verb, gre has raw and forward.  none.ndb did not
mean "no network at all", though the manual said it did.  It is now a
whitelist of control messages that cannot reach the network by
themselves, which is the argument this code already made about ndb
attributes it does not recognise, applied where it was not.

Also blocking: %M was never installed, so fmtrules emitted ipmask=%M% and
every ctl edit on a rule set containing ip= failed, while save wrote a
file reload would reject.  Tests had exercised the ctl path and the ip=
path but never together.

parserules built the new list in the globals with no lock, so for the
length of a reload the relay procs walked a list that was empty and then
half built -- exactly what installrules' comment promised could not
happen.  etherwriteip put a 64KB frame on a 32KB proc stack, the same bug
design.md records learning and fixing in relay().  putback and
notehandler were dead code: atexit matches on the registering pid and
_exits never runs the handlers, which is why the cleanup "did not fire"
rather than being flaky.  Nothing puts the card back, and the docs that
said otherwise are corrected.

The rest: expired flows kept matching and refreshing themselves; the pkt
interface claimed a 4096 MTU from a 1514-byte card; ports were read out
of non-first fragments; /net/ndb and /net/log were writable and
ipifc/*/data readable through the filter; control files were
world-writable, and owning them as a user called "fw" locked out the
administrator instead; delete 0 appended a rule reading <nil>; a dead
relay left one direction unfiltered with nothing to notice; and IPv6
unicast under -e was dropped in silence when it is simply not
implemented.

All three modes regression tested after: a namespace refusing headers and
port 22 while allowing 443, a card passing https and then blocking it
live, and the machine's network restored afterwards.

doc/todo.md says which of these were reproduced and which were read.

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

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