net: wget - real HTTP over the link port via the gateway

Grow the link-port demo from echo to actual network access. The Game Boy
still only does SLIP framing + display; the host gateway does DNS/TCP/HTTP.

- c/netlib.h: SLIP framing factored out (header-only, per-program copy).
  necho.c now uses it too.
- c/wget.c: `wget URL` frames the URL, then prints the reply body. The
  gateway streams the body back as typed frames: 'D'<chunk> ... 'E'.
- tools/gateway.py: add --mode http (urlopen the frame as a URL, cap the
  body, chunk it) alongside --mode echo; --cmd runs any gbos command.

Verified: `wget example.com` streams back the full Example Domain HTML onto
the terminal; `wget sl0p.foo` fetches the real page. A Game Boy on the web,
over the link cable.

1 files changed, 39 insertions, 0 deletions

diff --git a/c/netlib.h b/c/netlib.h
new file mode 100644
index 0000000..ec98eac
--- /dev/null
+++ b/c/netlib.h
@@ -0,0 +1,39 @@
+#ifndef NETLIB_H
+#define NETLIB_H
+/* SLIP (RFC 1055) framing over the link port. Header-only so each program gets
+   its own copy (the C build links one translation unit per program). */
+#include "gbos.h"
+
+#define SLIP_END     0xC0
+#define SLIP_ESC     0xDB
+#define SLIP_ESC_END 0xDC
+#define SLIP_ESC_ESC 0xDD
+
+/* send a framed packet (len up to 255) */
+static void slip_send(const char *buf, unsigned char len) {
+    unsigned char i, c;
+    ssend(SLIP_END);
+    for (i = 0; i < len; i++) {
+        c = (unsigned char)buf[i];
+        if (c == SLIP_END)      { ssend(SLIP_ESC); ssend(SLIP_ESC_END); }
+        else if (c == SLIP_ESC) { ssend(SLIP_ESC); ssend(SLIP_ESC_ESC); }
+        else                      ssend(c);
+    }
+    ssend(SLIP_END);
+}
+
+/* receive one framed packet into buf (max), returns its length */
+static unsigned char slip_recv(char *buf, unsigned char max) {
+    unsigned char len = 0, c;
+    for (;;) {
+        c = srecv();
+        if (c == SLIP_END) { if (len) return len; continue; }
+        if (c == SLIP_ESC) {
+            c = srecv();
+            if (c == SLIP_ESC_END) c = SLIP_END;
+            else if (c == SLIP_ESC_ESC) c = SLIP_ESC;
+        }
+        if (len < max) buf[len++] = (char)c;
+    }
+}
+#endif