*(home): do not require root privileges on the first run

Instead of requiring root privileges, we now check if AdGuard Home can
bind to privileged ports. If it cannot, we suggest either running it
with root privileges or grant CAP_NET_BIND_SERVICE capability. Please
note, that on Windows we still require root access.

Closes: #1699

util/net.go

@@ -0,0 +1,21 @@
+package util
+
+import (
+	"fmt"
+	"net"
+)
+
+// CanBindPort - checks if we can bind to this port or not
+func CanBindPort(port int) (bool, error) {
+	addr, err := net.ResolveTCPAddr("tcp", fmt.Sprintf("127.0.0.1:%d", port))
+	if err != nil {
+		return false, err
+	}
+
+	l, err := net.ListenTCP("tcp", addr)
+	if err != nil {
+		return false, err
+	}
+	_ = l.Close()
+	return true, nil
+}