Enhancement: Use config file permissions to control file group access

Previously files in a local/SFTP repository would always end up with very
restrictive access permissions, allowing access only to the owner. This
prevented a number of valid use-cases involving groups and ACLs.

We now use the permissions of the config file in the repository to decide
whether group access should be given to newly created repository files or
not. We arrange for repository files to be created group readable exactly
when the repository config file is group readable.

To opt-in to group readable repositories, a simple `chmod -R g+r` or
equivalent on the config file can be used. For repositories that should
be writable by group members a tad more setup is required, see the docs.

Posix ACLs can also be used now that the group permissions being forced to
zero no longer masks the effect of ACL entries.

https://github.com/restic/restic/issues/2351
https://github.com/restic/restic/pull/3419
https://forum.restic.net/t/1391
