Support /var/run being a symlink in upload

Support /var/run being a symlink in upload

Starting from Debian Wheezy, /var/run is a symlink to /run. If rsync
does not treat receiving symlinks to directories as directories, it
would remove the original symlink and create a new, empty directory
where only the files to sync are put in. In the case of /var/run, this
behavior severly impacts the running system (as, for example, several
PID can't be found anymore).

This is a cherry-pick of commit 412e738.

Signed-off-by: Thomas Thrainer <thomasth@google.com>
Reviewed-by: Klaus Aehlig <aehlig@google.com>