fix: Windows/MSYS2 build compatibility

- proxy/icmp_proxy.c: define struct icmp/ip + ICMP constants for _WIN32
- lwip_tcp_priv.h: use #include <winsock2.h> instead of #define htonl/htons
- lwip_tcp.h/c: rename poll field to pollfn (avoid WSAPoll macro conflict)
- tests: guard Unix-only headers with #ifndef _WIN32, skip fork/socketpair tests

src/proxy/icmp_proxy.c

@@ -19,6 +19,29 @@
 #include <netinet/ip_icmp.h>
 #include <arpa/inet.h>
 #include <sys/socket.h>
+#else
+#define ICMP_MINLEN      8
+#define ICMP_ECHO        8
+#define ICMP_ECHOREPLY   0
+struct icmp {
+    uint8_t  icmp_type;
+    uint8_t  icmp_code;
+    uint16_t icmp_cksum;
+    uint16_t icmp_id;
+    uint16_t icmp_seq;
+    uint8_t  icmp_data[1];
+};
+struct ip {
+    uint8_t  ip_hl:4, ip_v:4;
+    uint8_t  ip_tos;
+    uint16_t ip_len;
+    uint16_t ip_id;
+    uint16_t ip_off;
+    uint8_t  ip_ttl;
+    uint8_t  ip_p;
+    uint16_t ip_sum;
+    struct in_addr ip_src, ip_dst;
+};
 #endif
 
 #define ICMP_DEF_TTL        64