net: a TCP/IP stack, and AF_INET on the sockets that were already there

lwIP 2.2.0, fetched by ports/lwip and compiled straight into the kernel.
Not written from scratch, and the reason is not the packet formats: it is
congestion control, retransmission, window management and twenty years of
corner cases that only show up against a real peer. Not FreeBSD's
netinet either, though the licence would have allowed it -- that stack is
not a library, it reaches into mbufs, epoch(9), netisr, VNET and uma(9),
so adopting it means porting FreeBSD's kernel infrastructure first. lwIP
was written for exactly this situation: a small kernel with a raw
Ethernet driver and a port layer of a few hundred lines.

It is kernel code, so nothing is installed into the sysroot and there is
no second build system: the top-level Makefile compiles the files it
wants against our port headers. The file list is explicit rather than a
wildcard, because lwIP ships IPv6, 6LoWPAN, PPP, SLIP, DNS-SD and a dozen
apps in the same tree, and building what is not configured yields either
dead code or a link error naming a port function nobody meant to write.
Its sources are unmodified and its warnings are silenced for its files
only -- patching them would mean maintaining a fork.

The port layer is ours: a lock, a clock, the netif that hands frames to
e1000, lwipopts.h, and a shim/ directory of tiny headers so lwIP's
<string.h> and friends resolve to the kernel's functions instead of the
host's, whose prototypes disagree and would otherwise be found on the
default include path.

NO_SYS=1, so lwIP has no threads of its own and one kernel thread feeds
it. A thread rather than the interrupt handler, because a call into lwIP
can reach the driver, the driver can sleep, and an interrupt handler
cannot -- so the interrupt still only clears the cause register and the
thread polls. kthread_create is new and generally useful: a process with
a kernel-only page directory, no files, no cwd, no parent, dispatched
from forkret, which can therefore sleep and be preempted like anything
else. DHCP obtains an address at boot and the console says which one.

AF_INET went in exactly where phase 2 said it would. socknew dispatches
on the domain, kernel/net/inet.c implements the internet side on lwIP's
raw API, and not one syscall changed shape: bind, connect, listen,
accept, read and write reach the same descriptors they always did. TCP
and UDP both work. cmd/nettest opens a connection, writes, and reads the
answer back.

Two things are owed and named here so they are not discovered later:
recvfrom does not yet report the source address, and the socket paths
poll on the timer tick instead of sleeping on the socket. Both are
follow-ups, not redesigns.

NOTICE carries lwIP's BSD-3 text. Clause 3 is no-endorsement, not the
old advertising clause, so it sits with Apache-2.0 without difficulty.

Verified: the tree builds clean, and the headless boot check passes with
27 [ok] and no failures with the stack compiled in and the thread
running -- which is the part that would break first if netinit deadlocked
or the thread starved anything. An end-to-end connection test belongs in
that suite and is the next commit.

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

include/defs.h

@@ -379,9 +379,30 @@ int             sock_hup(struct socket*);
 int             sock_shutdown(struct socket*, int how);
 int             sockpair(struct socket **a, struct socket **b);
 const char*     sockname(struct socket*);
+// AF_INET addressing. Separate from sockbind/sockconnect because the families
+// do not name the same kind of thing; `addr` and `port` are network order.
+int             sockbind_in(struct socket*, uint addr, ushort port);
+int             sockconnect_in(struct socket*, uint addr, ushort port);
+int             sock_domain(struct socket*);
+
+// net/inet.c -- AF_INET on lwIP's raw API. Called only from net/unix.c, which
+// is where the per-domain dispatch lives.
+void*           inet_new(int type);
+int             inet_bind(void *h, uint addr, ushort port);
+int             inet_listen(void *h, int backlog);
+void*           inet_accept(void *h);
+int             inet_connect(void *h, uint addr, ushort port);
+int             inet_read(void *h, char *buf, int n);
+int             inet_write(void *h, char *buf, int n);
+int             inet_readable(void *h);
+int             inet_hup(void *h);
+void            inet_close(void *h);
 
 // proc.c
 void            exit(int status);
+// Start a kernel thread: a process that runs `fn` in kernel mode forever. It
+// has no files, no cwd and no parent, and must never return. Returns the pid.
+int             kthread_create(void (*fn)(void), const char *name);
 int             fork(void);
 int             growproc(int64);
 int             kill(int);
@@ -504,6 +525,20 @@ int             diskwrite(struct inode*, uint64, char*, int);
 int             e1000_transmit(const void *frame, int len);
 int             e1000_receive(void *out, int cap);
 const unsigned char *e1000_mac(void);
+int             e1000_present(void);
+
+// net/net.c -- the TCP/IP stack (lwIP). netinit starts the network thread;
+// net_is_up reports whether an address has been obtained.
+void            netinit(void);
+int             net_is_up(void);
+
+// net/lwip/ -- the port layer and the interface, called only from net.c.
+void            lwip_sys_init(void);
+void            lwip_sys_lock(void);
+void            lwip_sys_unlock(void);
+int             net_e1000_attach(void);
+int             net_e1000_poll(void);
+struct netif   *net_e1000_netif(void);
 void            e1000_selftest(void);
 
 // registry.c