test: the network is checked by the suite now, and /proc/netif says what it did

The TCP/IP stack landed with its end-to-end test performed by hand: start
a listener, boot, type nettest, read the output. That is evidence once
and nothing afterwards, and it is exactly the kind of check that quietly
stops being run.

bootcheck -Net does the whole exchange. A listener runs on this host on a
port picked by binding one and letting go, the guest is told to connect
out to 10.0.2.2 at that port, and both ends are asserted: the guest
reports what it received, the listener reports what arrived. One side
alone proves much less than it looks -- a stack that completes a
handshake and then drops every byte satisfies any check made from inside
the guest.

The listener is a background job, a separate process, because this
harness has one hard rule: nothing may block the writer feeding QEMU's
stdin. A socket accept is precisely the wait that deadlocked two earlier
attempts at reading QEMU's output. It answers the protocol nettest
speaks and then closes -- the close is load-bearing, since nettest reads
until end of file and a listener that stayed open would hang the guest
rather than fail it.

The NIC is spelled out (-netdev user + -device e1000) rather than left to
QEMU's default. Other runs can take the default and still answer "did a
driver bind"; a run that asserts on an address should be reading its own
configuration.

/proc/netif is new, and the address is why. Until now the only report was
a line on the console at boot, which userland cannot read and a test can
only find by scraping the transcript. It gives interface, MAC, state,
address, netmask, gateway, MTU and frame counts. Named netif rather than
net on purpose: /proc/net is a *directory* on Linux, and a plain file
there would give ENOTDIR to anything reaching for /proc/net/dev, which is
worse than not having it. The counters are kept at the lwIP seam rather
than in the driver, because a frame the driver received and the netif
dropped never reached the stack.

Verified: 38 [ok], no failures. Eleven of them are new -- lwIP up, DHCP
obtaining 10.0.2.15 from SLIRP (a number nothing in the guest knows in
advance, so it is the exchange completing rather than a guess), the four
/proc/netif fields, frames counted in both directions, the handshake, the
reply read back, and the listener confirming HELLO-FROM-SCHOS arrived.

One bug found by writing it: $netPort was chosen after the command list
was built, so the guest ran `nettest 10.0.2.2` with no port and printed
its usage. An unset PowerShell variable interpolates to nothing rather
than failing, which is worth remembering next time a command in this
file looks right and behaves oddly.

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

include/defs.h

@@ -531,6 +531,7 @@ int             e1000_present(void);
 // net_is_up reports whether an address has been obtained.
 void            netinit(void);
 int             net_is_up(void);
+int             net_info(char *buf, int cap);
 
 // net/lwip/ -- the port layer and the interface, called only from net.c.
 void            lwip_sys_init(void);
@@ -538,6 +539,7 @@ void            lwip_sys_lock(void);
 void            lwip_sys_unlock(void);
 int             net_e1000_attach(void);
 int             net_e1000_poll(void);
+void            net_e1000_counts(uint64 *rx, uint64 *tx);
 struct netif   *net_e1000_netif(void);
 void            e1000_selftest(void);