fix: Global flock file accessible by every user / Introducing FlockContext manager

[buhtz]

• Introduce a context manager to handle (exclusive) file locks.
• Move global flock file from /tmp to /run/lock to make it accessible for the root user. The background is that, because of a quit fresh Linux feature, the "root" user is not allowed to write into any file in /tmp no matter if file permissions would allow it.
• There are much more other flock mechanics in BIT but I didn't touch them yet. Should be done in a separate PR.
• The method Snapshots.backup() is the only one using the global flock. The method smells like hell. There are ~170 lines between the lock and its release. And I decided not to touch this in this PR. So I just indented that lines to use them inside the new context manager.

It is an idea.

Might...
Close #1676
Close #1122

EDIT: One of the two reporters tested and confirmed the PR.

[buhtz]

Not the only possible method but one of the recommended methods how to add copyright and license info to each file in a repo.

Further reading: REUSE Software and SPDX (ISO/IEC 5962:2021) specifications

[buhtz]

Other naming suggestions? GlobalFileLock, ...?

[buhtz]

This might have caused bugs in the past. The flock was never released when this "return" took affect. Now with the context manager the flock is always released.

[buhtz]

Here the call to releaseFlock() was also missing.

[aryoda]

SDPX looks quite promising to me to support machine-readable licenses too

[buhtz]

Hope it is OK to now treat is as reviewed?

I will also close the two related issues with merging this.