[BUGFIX] Try to always honor local config for mounts

[AnomalRoil]

I realize this is a big PR, but sadly the more I was digging to fix #2721, the more I would uncover new edge cases around our local vs global config handling 😢

This:

• adds a new config.WithMount that allows to pass to a context the current "mount point" or "store" that we're working on.
• changes config.FromContext to also return the mount point if any is provided in the context.
• changes config.Int, Bool and String to take into account the mount point if any is provided.
• attempts to uses WithMount in the relevant places, mostly: Show and Generate are concerned by custom config options, as well as anything that can use the clipboard feature.

Furthermore, in "config related" changes, this also:

• documents the config options that aren't supported at the local level.
• prevents local configs from tampering with security features such as recipients.hash.
• fixes recipients ack to actually update the recipients.hash when it was previously outdated, but the .gpg-id were correct.
• adds a few convenience functions to our internal config package.

In the "weird changes" I still did in here, this also:

• fixes a few typos.
• aligns the use of <root> to indicate the local root store when using the --store flag even in sync.
• fixes a race condition in rcs_test.go where git was weirdly failing with failed to add "/tmp/TestInitRCS2737126547/001/git" to git: exit status 128: fatal: Unable to create '/home/anomalroil/code/gopass/.git/index.lock.lock': File exists. because of some Parallel tests. This might actually indicate a bigger underlying issue: I would expect our tests not to tamper with the local git things, and do everything in /tmp....

Let me know if you'd like me to chunk it into multiple PR / commits, it just was a big rabbit hole and it kept on giving more.

This is probably not entirely done yet, e.g. I don't think core.autopush is properly respected at the local level yet, might be worth opening a new issue to track a big review of all our config options?

[dominikschulz]

Amazing, thanks a lot!