Revert to 'ProtectSystem=strict' in boinc-client.service and make '/tmp' writable

[computezrmle]

BOINC volunteers running VirtualBox tasks on Linux occasionally report errors like this if BOINC runs as a systemd service:

2024-12-28 01:20:15 (16867): Detected: VirtualBox VboxManage Interface (Version: 7.0.12)
2024-12-28 01:20:20 (16867): Error in host info for VM: -182
Command:
VBoxManage -q list hostinfo 
Output:
VBoxManage: error: Failed to create the VirtualBox object!
VBoxManage: error: Code NS_ERROR_SOCKET_FAIL (0xC1F30200) - IPC daemon socket error (extended info not available)
VBoxManage: error: Most likely, the VirtualBox COM server is not running or failed to start.


2024-12-28 01:20:20 (16867): WARNING: Communication with VM Hypervisor failed.
2024-12-28 01:20:20 (16867): ERROR: VBoxManage list hostinfo failed
2024-12-28 01:20:20 (16867): called boinc_finish(1)

Usually this can be tracked down to a too restrictive setting in BOINC's systemd file:
ProtectSystem=strict

In most cases experienced volunteers suggest to replace strict with full, run systemctl daemon-reload and restart BOINC.
This works since - unlike strict - full leaves /tmp writable for BOINC and it's child processes.
The disadvantage is that it also leaves many other directories writable.
A better solution might be to leave strict and make /tmp writable which can be configured using ReadWritePaths=.

Background details

From the systemd manual

ProtectSystem=
Takes a boolean argument or the special values "full" or "strict". If true, mounts the /usr/ and the boot loader directories (/boot and /efi) read-only for processes invoked by this unit. If set to "full", the /etc/ directory is mounted read-only, too. If set to "strict" the entire file system hierarchy is mounted read-only, except for the API file system subtrees /dev/, /proc/ and /sys/ (...) If this option is used, ReadWritePaths= may be used to exclude specific directories from being made read-only.

From the VirtualBox manual

VBoxSVC IPC Issues
On Linux, Oracle VirtualBox makes use of a custom version of Mozilla XPCOM (cross platform component object model) for interprocess and intraprocess communication (IPC). (...) Communication between the various Oracle VirtualBox components and VBoxSVC is performed through a local domain socket residing in /tmp/.vbox-username-ipc. In case there are communication problems, such as an Oracle VirtualBox application cannot communicate with VBoxSVC, terminate the daemons and remove the local domain socket directory.

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• client/scripts/boinc-client.service.in: Language not supported

[AenBleidd]

@Germano0, @lfielf, could you please review this PR?
@computezrmle, please fix the tests (IF you don't want or don't know how - I can do that for you)

[computezrmle]

@computezrmle, please fix the tests ...

Done.