gphoto2: Return the correct offset when seeking

https://bugzilla.gnome.org/show_bug.cgi?id=712601
dav: Report number of bytes used on the filesystem

Report the number of bytes used by exposing the "quota-used-bytes"
property.

https://bugzilla.gnome.org/show_bug.cgi?id=712654
sftp: Implement try_query_fs_info using the OpenSSH statvfs extension

Use the statvfs command from OpenSSH to fill in the filesystem size,
free and used attributes as well as whether the filesystem is readonly
or not.

The extension is defined at http://api.libssh.org/rfc/PROTOCOL.
The extension is signified by the name "statvfs@openssh.com" with a
version of "2".

Based on a patch Alban Browaeys.

https://bugzilla.gnome.org/show_bug.cgi?id=549207