as root on posix try to retain repository owner uid/umask

[RonnyPfannschmidt]

when running borg create as root (for example to back up all home folders)
then borg will create the new files owned as root and brick the repository for use by the original owner because the next cache sync will fail to read the created segments

as current workaround i have to run chown -R ronny:ronny backups after each backup

in order to elevate that, borg should try to use the owning uid/gid of the backup folder if it is allowed to
(so at least root an do correct backups)

[ThomasWaldmann]

Workaround:

• even when doing local backups, use wanteduser@localhost:repo.
• optionally, use BORG_RSH additionally to avoid everything going via ssh.

[ThomasWaldmann]

As there was no work on this, a workaround exists and we are close before RC phase, I removed this from this milestone.

[ThomasWaldmann]

Closing this:

• no change planned for borg 1.x.
• for borg2, borgstore is used for storage.
• this feature would need additional OS calls to "fix" owner/group (if different from current process uid/gid). not sure if that is worth it if one can just always run borg as the same user.