Add options for handling attempt to create new archive in a mounted repo

[v12mike]

I have a couple of times been doing site maintenance or testing that required that a borg repo be mounted to extract some files and while the repo was mounted the scheduled backup to that repo failed to run due to the repo being mounted.

I would like an option that either :

1. Allows an archive to be created while the repo is mounted. (or if that is difficult)
2. Automatically unmounts a repo if an attempt is made to add an archive to the mounted repo.

[ThomasWaldmann]

1. is difficult, borg locks the repo against parallel usage while working with it.
2. is not possible either, because borg mount does not know of any parallel access attempts.

the good news is that borg2 (master branch, beta, not for production) does not need that kind of locking.

[ThomasWaldmann]

For borg 1.x you can use borg mount -f ... to avoid backgrounding the process, so you maybe don't forget to unmount. The terminal will be blocked then, so you'll need another terminal to work with the mounted archive.