[SoC2020] Support upgrading the existing D-Bus daemon without reboot

# 项目名称/Project Title
D-Bus: 支持热升级
Support upgrading the existing D-Bus daemon without reboot

# 项目描述/Project Description
D-Bus 是一个用于进程间通信的轻量级的消息总线系统，其官网链接：https://www.freedesktop.org/wiki/Software/dbus/
D-Bus 除了应用于桌面环境，现在也应用于服务器环境中。服务器作为信息时代的基石，需要 7*24 小时不间断运行，D-Bus 作为 systemd 的一个服务，也需要能够长期稳定运行。但是目前 D-Bus 不支持热升级，即升级 D-Bus 软件包之后，需要重启服务器。社区在这方面也有一些讨论：
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=805449
本项目计划添加对 D-Bus 热升级的支持。

D-Bus is a lightweight message bus system for inter-process communication, and its official website link: https://www.freedesktop.org/wiki/Software/dbus/
D-Bus is used not only in the desktop environment, but also in the server environment. As the cornerstone of the information age, the server needs 7 * 24 hours of uninterrupted operation. As a service of systemd, D-Bus also needs to be able to run stably for a long time.
However, after upgrading the D-Bus software package, we need to restart the server.
The community also has some discussions:
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=805449
This project plans to support upgrading the existing D-Bus daemon without reboot.

# 预期成果/Expected Outcomes
D-Bus支持热升级之后，修复 D-Bus 的 CVE 漏洞或者 BUG 后，不用重启系统，有助于 Alibaba Cloud Linux 2 成为运行时间最长、最稳定的系统。

After supporting the upgrade of the existing dbus daemon without reboot, we can fix some D-Bus cve vulnerabilities or bugs online, which will help Alibaba Cloud Linux 2 become the longest running and most stable system.

# 需要的技能/Required Skills
- 熟悉 C/C++ 语言编程
- D-Bus 的背景知识

- Familiar with C
- D-Bus related background knowledge

Start with the D-Bus project description and the linked Debian bug discussion to determine the required daemon upgrade behavior. Success means upgrading the existing D-Bus package without rebooting and keeping the service running while fixes are applied; the issue names no repository files, entry points, or tests.

Written by the indexing model from the issue text.