net: a network *class*, so the stack stops knowing what an e1000 is

The TCP/IP stack reached straight into the first driver anyone wrote.
net.c called e1000_present(), the lwIP glue was named netif_e1000.c and
called e1000_transmit(), and include/defs.h declared the driver's
internals to the whole kernel. The second NIC driver would therefore
have arrived with a near-identical copy of the glue file and a patch to
the protocol stack, which is the arrangement include/device.h was
written to prevent -- the e1000 even said so, with `.ops = 0, // no
class vtable yet; there is no net class`.

There is now. A driver publishes a struct netdev_ops -- transmit,
receive, mac, link_up -- and is finished: it never mentions lwIP, and
nothing above it learns its name. The display class was already this
shape; this is the same shape for networking, and the two now read the
same way in kernel/dev/device.c.

What that made possible immediately:

  - One netif per bound device instead of one per driver. lwIP's glue is
    netif_eth.c, it takes a struct device, and it holds up to NETIF_MAX
    interfaces. net.c attaches every network device it finds and starts
    DHCP on each, because which interface the network answers on is not
    ours to decide.
  - /proc/netif lists all of them, one block of fields each, with the
    interface name and the driver that claimed it.
  - The ARP self-test moved from the e1000 to the class. Nothing in it
    was chip-specific, and it is the first thing anyone will want to run
    against the next driver. Still `echo n > /proc/sysrq`.
  - Private headers live with their driver: e1000.h was in include/,
    where exactly one file could use it. The driver build now adds the
    driver's own directory to the include path.

The e1000 also drives more than one card. Its probe() refused the second
outright, because there was a single static instance -- so a machine
with two of them drove one and silently ignored the other. Instances are
an array now, and the instance is hung on dev->priv by a new optional
`attached` hook that the bus calls once the device exists. probe() runs
before device_add(), so there was previously nowhere to put it; that is
the whole reason the hook exists rather than a line inside probe.

QEMU gets an explicit NIC on every target (QEMU_NET). The default device
happens to be an e1000 on this machine type today, which is exactly the
sort of thing that stays true until a QEMU release changes it and the
network stops being exercised without anyone noticing.

Verified:
  - bootcheck -Net passes, 48 [ok], no failures -- including the moved
    self-test and two new checks that /proc/netif names both the
    interface and the driver behind it, which together say the stack
    went through the class rather than through a driver it was compiled
    to know.
  - Two NICs attached by hand (-device e1000 twice, second with its own
    MAC): eth0 and eth1 both attach, each reports its own MAC in
    /proc/netif, and each gets its own DHCP lease. Before this commit
    the same machine produced one interface.
  - make schfs-check still passes in full.

One trap worth recording: moving a header broke the build for anyone
with stale .d files -- make stopped with "No rule to make target
'include/e1000.h'". CFLAGS gains -MP, which emits a phony target per
header so a moved or deleted one can never wedge the build again.

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

kernel/dev/procfs.c

@@ -21,6 +21,7 @@
 #include "x86.h"
 #include "device.h"
 #include "display.h"
+#include "netdev.h"
 #include "param.h"
 #include "memlayout.h"
 #include "mmu.h"
@@ -444,7 +445,7 @@ proc_sysrq_cmd(char cmd)
     display_selftest();
     break;
   case 'n':
-    e1000_selftest();
+    netdev_selftest();
     break;
   case 't':
     text_selftest();