shared repository content file permissions for v9

shared repository content file permissions for v9

v9 will not need to write to annex content files in order to lock them,
so freezeContent removes the write bit in a shared repository, the same
as in any other repository.

checkContentWritePerm makes sure that the write perm is not set, which
will let git-annex fsck fix up the permissions. Upgrading to v9
will need to fix the permissions as well, but it seems likely there will
be situations where the user git-annex is running an upgrade as cannot,
so it will have to leave the write bit set. In such a case, git-annex
fsck can fix it later.

Sponsored-by: Dartmouth College's Datalad project