feat(dns): use system nameservers from /etc/resolv.conf as fallback

When no forwarder or dns metadata is configured, the DNS handler now
reads system nameservers from /etc/resolv.conf (Unix) before falling
back to the hardcoded 127.0.0.1:53 default. This makes the DNS proxy
work out-of-the-box without requiring explicit DNS server configuration.

Fixes go-gost/gost#89

handler/dns/resolv_unix.go

@@ -0,0 +1,50 @@
+//go:build !windows
+
+package dns
+
+import (
+	"bufio"
+	"fmt"
+	"net"
+	"os"
+	"strings"
+)
+
+// systemNameservers reads the system DNS configuration from /etc/resolv.conf
+// and returns the nameserver addresses as URL-like strings suitable for
+// exchanger.NewExchanger (e.g., "udp://1.1.1.1:53").
+// Returns nil if the file cannot be read or contains no nameserver entries.
+func systemNameservers() []string {
+	f, err := os.Open("/etc/resolv.conf")
+	if err != nil {
+		return nil
+	}
+	defer f.Close()
+
+	var servers []string
+	scanner := bufio.NewScanner(f)
+	for scanner.Scan() {
+		line := strings.TrimSpace(scanner.Text())
+		if line == "" || line[0] == '#' || line[0] == ';' {
+			continue
+		}
+		fields := strings.Fields(line)
+		if len(fields) < 2 || fields[0] != "nameserver" {
+			continue
+		}
+		addr := fields[1]
+		// Verify it's a valid IP address.
+		if ip := net.ParseIP(addr); ip == nil {
+			continue
+		}
+		// If no port, default to 53.
+		if _, _, err := net.SplitHostPort(addr); err != nil {
+			addr = net.JoinHostPort(addr, "53")
+		}
+		servers = append(servers, fmt.Sprintf("udp://%s", addr))
+	}
+	if err := scanner.Err(); err != nil {
+		return nil
+	}
+	return servers
+}