net: DHCP client - lease the IP at boot instead of hardcoding it

The address is no longer baked in. net_init starts at 0.0.0.0; a DHCP client
runs as the first thing on boot (init/shell forks it and waits), and only once
it has a lease (or gives up) does the prompt appear.

Kernel:
- IP is configurable: 0.0.0.0 until leased; NET_SETIP op stores it.
- 16-bit frame length. DHCP/BOOTP packets are ~272 bytes, over the old 255-byte
  frame cap, so net_slip_send takes a 16-bit length, net_pump assembles into a
  320-byte buffer with a 16-bit wNetRxLen, and udp_send writes a 16-bit IP total.
  Payloads stay <=255 (kept small on purpose) so the per-protocol datalen math
  is unchanged. wNetTx/wNetRxBuf 256->320, SK_RXBUF 208->288.

Userland:
- c/dhcp.c: DISCOVER->OFFER->REQUEST->ACK over a UDP socket, then net_setip();
  times out gracefully (shell still boots) if there's no server. sh.c runs it
  before the prompt.

Bridge (self-contained DHCP server, no dnsmasq):
- tunbridge.py + netboot intercept UDP->:67 and answer OFFER/ACK leasing
  10.0.0.2 (gateway 10.0.0.1); everything else is bridged/NATed as before.

Regression-tested ICMP/UDP/TCP after the 16-bit change. Verified end to end:
    dhcp: discovering
    dhcp: leased 10.0.0.2
    /# ping 10.0.0.1   -> 4/4 (traffic from the leased address)

1 files changed, 6 insertions, 0 deletions

diff --git a/c/sh.c b/c/sh.c
index a8dd09d..3178894 100644
--- a/c/sh.c
+++ b/c/sh.c
@@ -155,6 +155,12 @@ void main(void) {
     char *tok[16];
     char cwd[40];
     cwd[0] = '/'; cwd[1] = 0;
+    /* bring up networking first: run the DHCP client and wait for it to finish
+       (got a lease, or gave up). Only then drop to the prompt. */
+    {
+        unsigned char d = lookup("dhcp");
+        if (d != NOFD) { if (fork() == 0) exec(d); else wait(); }
+    }
     for (;;) {
         unsigned char nt, i, pipepos, c, bg;
         /* reap finished background jobs */