sftp: Allow seek past end of file

also, don't query for the file size unless required for SEEK_END

https://bugzilla.gnome.org/show_bug.cgi?id=709432
mtp: Allow seek past end of file

https://bugzilla.gnome.org/show_bug.cgi?id=709432
cdda: Allow seek past end of file

https://bugzilla.gnome.org/show_bug.cgi?id=709432
gphoto2: Allow seek past end of file

https://bugzilla.gnome.org/show_bug.cgi?id=709432
afp: Allow seek past end of file

Also, only query file size if needed for G_SEEK_END.

https://bugzilla.gnome.org/show_bug.cgi?id=709432