What does this PR change? What problem does it solve?

Restic has historically relied on a homegrown mechanism using cleanup handlers to handle command interrupts like SIGINT. This PR completely removes that mechanism and only uses Contexts to cancel commands. This removes another source of global state from cmd/restic/ and might be useful in the future as it allows for a controlled shutdown of commands compared to the current approach that is more akin to a "controlled crash".

Due to the cleanup handlers approach there was no need to ensure that every command exits as fast as possible once a context has been canceled. This PR therefore adds lots of checks for canceled contexts. Previously, if the cleanup handlers took too long, then a command like prune could in some cases erroneously report a corrupted repository.

From a user perspective there shouldn't be any large changes, although the text output of an interrupted command will change slightly.

There are two ugly parts in the current implementation: reading a password and serving a fuse mount appear to offer no nice way to honor a canceled context. To still somewhat integrate with context cancellation, the blocking command runs in a separate goroutine that is leaked if the command gets canceled. This is good enough for uses in restic, as canceling the context will result in program termination anyways.

Was the change previously discussed in an issue or on the forum?

Mentioned as part of the restic 0.17.0 roadmap. Prerequisite for #4406 .

Checklist

• I have read the contribution guidelines.
• I have enabled maintainer edits.
• I have added tests for all code changes.
• [ ] I have added documentation for relevant changes (in the manual).
• [ ] There's a new file in changelog/unreleased/ that describes the changes for our users (see template).
• I have run gofmt on the code in all commits.
• All commit messages are formatted in the same style as the other commits in the repo.
• I'm done! This pull request is ready for review.