Add bare-user-only repo mode

Add bare-user-only repo mode

This mode is similar to bare-user, but does not store the permission,
ownership (uid/gid) and xattrs in an xattr on the file objects in the
repo. Additionally it stores symlinks as symlinks rather than as
regular files+xattrs, like the bare mode. The later is needed because
we can't store the is-symlink in the xattr.

This means that some metadata is lost, such as the uid. When reading a
repo like this we always report uid, gid as 0, and no xattrs, so
unless this is true in the commit the resulting repository will
not fsck correctly.

However, it the main usecase of the repository is to check out with
--user-mode, then no information is lost, and the repository can
work on filesystems without xattrs (such as tmpfs).

Closes: #750
Approved by: cgwalters