ether: keep the frame buffer off the proc stack, and log the v6 drop

Maxframe was Ehdrlen + 16K, an automatic in etherwriteip, which runs on
a proc created with a 32K stack.  It fits, and the previous fix was
right that 64K did not, but half a stack for a buffer that -- since the
pkt interface is now told "mtu 1500" -- can never hold more than 1514
bytes is a number waiting to be wrong again, and libthread allocates
that stack with malloc, so being wrong means quietly corrupting the
heap rather than faulting.

So the buffer belongs to the caller, with its size, and the guard is
against that size.  etherout allocates it once, from the same Maxpkt it
sizes its read buffer with, which is the only place that knows how much
can arrive.  ether.c no longer has a length of its own to drift.

The IPv6 message now also goes to syslog.  fw daemonizes, so a message
on file descriptor 2 goes wherever the shell that started it was
pointing, which for a firewall started at boot is nowhere.

No test.  etherwriteip is reached only in card mode, which the suite
stays out of on purpose because a test that can leave the machine with
no network is a test nobody runs.  A unit harness for ether.c -- point
efd at a pipe, call etherwriteip, read the frame back -- would cover
this and the broadcast mapping that todo.md still records as written
but never observed.  Worth doing; not done here.

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

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