feat: Add socket-activated systemd service for discovery

[aleasto]

Introduce a system-wide daemon listening on /run/wsdd.socket, which is managed by systemd.
When a client first connects to the socket the daemon is started. Multiple clients can connect and safely use the API simultaneously.

The service runs in discovery-only mode.

Uses multicast source-port 37020.

[aleasto]

• Increases the security of services like gvfs which rely discovery mode by integrating with all the security confinement features
• Allows us to standardize one --source-port
• Only one daemon per system
• systemd-specific but non-intrusive

[aleasto]

Do we want to use the same flags from /etc/default/wsdd here?

[christgau]

Might help in some minor use-cases, I think (including verbosity for debugging, e.g.), so why not. But I propose to use a different environment file.

I also propose to use a different run/chroot directory for the wsdd-discovery service in order to separate it from a regular host instance. Maybe even a different (dynamic) username might be good, although I'm not sure if dynamic users are inherently distinct (due to different namespace) even when sharing the same name.

[aleasto]

although I'm not sure if dynamic users are inherently distinct (due to different namespace) even when sharing the same name.

They're using the same UID/GID here, so no

[aleasto]

How's this?
I've used the same env file but a different variable name

[aleasto]

--source-port 37020 could also be set in the environment file instead of hard-coded inthe systemd unit

[ondrejholy]

It would be nice to make that configurable from the environmental file, but it is hardcoded in the firewall configuration anyway...

[aleasto]

TODO: systemctl stop wsdd-discovery hangs if there are processes (gvfsd) listening on /run/wsdd.socket. We should probably close the connection.

[aleasto]

TODO: systemctl stop wsdd-discovery hangs if there are processes (gvfsd) listening on /run/wsdd.socket. We should probably close the connection.

That's #219