Resume development of alx, but at an earlier point than where I left
off. Specifically, trying to integrate with mii(4) caused problems that
I wasn't able to fix. So now my goal is to get things working first, and
_then_ try to use mii(4).

This commit adds some clumsy but functional support for rx. I'm now able
to ping from and to the AR8162 in my laptop!
Enable debug symbols for alx(4).
Refresh mbufs in the rx handler.
Some more progress on rx. Ping floods to a remote FreeBSD host that
isn't rate-limiting its ICMP replies eventually cause a hang.
Free mbufs after their contents have been transmitted this fixes the
ping flood hang: it occurred when the kernel ran out of mbufs, as alx
wasn't freeing them.
Fix a dumb bug: when freeing mbufs, we need increment the descriptor
index even if a buffer has a NULL mbuf pointer.

Also allow up to 32 mbuf fragments in the transmit. I have no idea what
the limit is; 32 just comes from other Atheros drivers in FreeBSD. With
this, the basics seem to be working well: ping floods don't cause any
drops, and I can ssh around and stuff. If all goes well, this commit
will be pushed using alx(4).
Merge branch 'no-mii'