[pass] Clip/restore concurrency issues (fix?)

[Stephen Blott]

A couple of issues relating to incorrect clip/restore concurrency issues have been raised recently.

The issues are described below.  However, fundamentally, the problem is that password-store is doing concurrent programming without any concurrency control.  Which doesn't work.

I've implemented a locking system for password-store based on flock(1).  It's here:

    - https://github.com/smblott-github/password-store/compare/locking

The approach is described in-line in the link above.

Here are the two problems which have been identified:

1. With two concurrent background processes, the second restores the first's password, leaving a password in the clipboard.

2. With two concurrent background processes, the clipboard is restored by the first before the 45 seconds of the second have elapsed.

Comments welcome.

Steve