When exporting a tree to a directory special remote, files are written mode
600. This prevents eg, publishing them by http, and then accessing them
with httpalso special remote.

`viaTmp` creates a temp file, with temp file perms. Either it should use
umask perms, or all callers that don't explicitly set perms should.

This also affects some other things, eg hook files written by git-annex
init, and some stuff in ~/.config/git-annex like autostart.

> [[fixed|done]] --[[Joey]]

`withTmpFileIn` also uses openTempFile, and probably its callers do need to
adjust perms if desired since it could be used with a real temp directory.

> Audited and fixed these. It affected only directory special remote and
> bittorrent special remote if a download from it were interrupted and then
> resumed by a different user than the one who started it. --[[Joey]]

There are also a couple of other uses of openTempFile, which need to be
audited for this problem. --[[Joey]]

> Checked, all were ok. --[[Joey]]
