net: an interface that appears after boot starts the stack

With the native r8169 out of the registry, the TUF A15 boots with no
network device; netinit() saw none and left the stack off, and
net_attach_late() then refused every interface that came later -- its
comment assumed netinit() was still to run. Linux's r8169 under LDLS
got as far as registering ldls0 and was turned away. A late interface
now starts the stack, whose first act is to attach every device there
is; net_eth_attach() refuses a device it already has, for the window
in which both could offer it.

LDLS says which refusal it got: EBUSY is another process, anything else
is the kernel.

Tested in QEMU with -nic none and netecho: no interface at boot, then
ldls0 with a DHCP lease and ping answered.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01S1giNvrHTrPMPw7V6x1kQ4

kernel/net/lwip/netif_eth.c

@@ -110,6 +110,11 @@ net_eth_attach(struct device *dev)
 
     if (nifs >= NETIF_MAX || dev == 0)
         return -1;
+    // Once only: a device that appears while netd is starting can be offered
+    // both by netd's boot-time walk and by net_attach_late.
+    for (int i = 0; i < nifs; i++)
+        if (ifs[i].used && ifs[i].dev == dev)
+            return -1;
 
     struct eth_if *e = &ifs[nifs];
     e->dev  = dev;