Fix Windows build: POSIX dependencies (netdb.h, fork, setsid, SIGHUP, etc.)

- Wrap POSIX headers (netdb.h, ifaddrs.h, net/if.h) in #ifndef _WIN32 in config_parser.c
- Add Windows stub for get_netif_index() - returns 0 (no interface binding)
- Wrap daemonize() function for Windows - returns 0 (no fork/setsid support)
- Replace setlinebuf() with setvbuf() for Windows compatibility
- Wrap SIGHUP signal handler in #ifndef _WIN32
- Add explicit poll() declaration for Windows in platform_compat.h
- All 22 tests pass on Linux

lib/platform_compat.h

@@ -103,6 +103,8 @@
     
     // nanosleep already defined in pthread_time.h on MSYS2
     // poll() and pollfd already defined in winsock2.h on MSYS2
+    // Explicitly declare poll() to avoid implicit declaration warning
+    int poll(struct pollfd *fds, unsigned long nfds, int timeout);
     
     // sa_family_t might not be defined on some Windows setups
     #ifndef sa_family_t

src/config_parser.c

@@ -9,9 +9,11 @@
 #include <ctype.h>
 #include "../lib/platform_compat.h"
 #include <errno.h>
+#ifndef _WIN32
 #include <netdb.h>
 #include <ifaddrs.h>
 #include <net/if.h>
+#endif
 
 #define MAX_LINE_LEN 1024
 #define INITIAL_ARRAY_CAPACITY 8
@@ -173,7 +175,13 @@ static int parse_address_and_port(const char *str, struct sockaddr_storage *sock
 
 static uint32_t get_netif_index(const char *ifname) {
     if (!ifname || strlen(ifname) == 0) return 0;
+#ifdef _WIN32
+    // Windows doesn't have if_nametoindex, return 0 to indicate no specific interface
+    (void)ifname;
+    return 0;
+#else
     return if_nametoindex(ifname);
+#endif
 }
 
 static struct CFG_CLIENT_LINK* create_client_link(struct CFG_SERVER* local_srv, const char *remote_addr) {

src/utun.c

@@ -159,6 +159,10 @@ static void remove_pidfile(const char *pidfile) {
 
 // Daemonize process
 static int daemonize(void) {
+#ifdef _WIN32
+    // Windows doesn't support fork/setsid, just return success
+    return 0;
+#else
     pid_t pid = fork();
     if (pid < 0) {
         perror("fork");
@@ -196,6 +200,7 @@ static int daemonize(void) {
     }
     
     return 0;
+#endif
 }
 
 // Open log file
@@ -208,8 +213,12 @@ static FILE* open_logfile(const char *logfile) {
         return stderr;
     }
     
-    // Set line buffering
+    // Set line buffering (Windows uses _IOLBF via setvbuf)
+#ifdef _WIN32
+    setvbuf(fp, NULL, _IOLBF, 0);
+#else
     setlinebuf(fp);
+#endif
     return fp;
 }
 
@@ -283,7 +292,9 @@ int main(int argc, char *argv[]) {
     // Setup signal handlers
     signal(SIGINT, signal_handler);
     signal(SIGTERM, signal_handler);
+#ifndef _WIN32
     signal(SIGHUP, signal_handler);
+#endif
     
     // Daemonize if not in foreground
     if (!args.foreground) {