fix: server shutdown on SIGTERM — use sigaction without SA_RESTART

The previous code used signal() to register the cleanup handler, but on glibc
(and many other modern libc implementations), signal() implicitly sets
SA_RESTART, which causes blocking syscalls like accept() to be transparently
restarted after the handler runs. As a result, accept() never returns EINTR,
the accept loop never breaks, and the server never exits on SIGTERM/SIGINT.

Fix by using sigaction() with sa_flags = 0 (no SA_RESTART), guaranteeing that
accept() is interrupted and returns EINTR, allowing the loop to exit and the
server to shut down cleanly.

src/server/server.c

@@ -180,8 +180,12 @@ int main(int argc, char* argv[]) {
     log_message(LOG_LEVEL_WARNING, "--ca has no effect without --tls");
   }
 
-  signal(SIGINT, cleanup);
+  struct sigaction sa;
-  signal(SIGTERM, cleanup);
+  sigemptyset(&sa.sa_mask);
+  sa.sa_handler = cleanup;
+  sa.sa_flags = 0; /* Do NOT set SA_RESTART — we need accept() to return EINTR */
+  sigaction(SIGINT, &sa, NULL);
+  sigaction(SIGTERM, &sa, NULL);
   g_server = server_create(port);
   if (g_server == NULL) {
     log_message(LOG_LEVEL_ERROR, "Failed to create server");