init: implement locking

init: implement locking

First, we use flock(2) to wait on parallel public-inbox-init(1)
invocations while we make multiple changes using git-config(1).
This flock allows -init processes to wait on each other if using
reasonable POSIX filesystems.

Then, we also need a git-config(1)-compatible lock to prevent
user-invoked git-config(1) processes from clobbering our
changes while we're holding the flock.