thumbnail: Handle non-usrmerged systems and non-existing directories
On systems where /usr-merge hasn't been carried out, /bin (etc) won't point to /usr/bin. In that case we should --ro-bind the directory instead of --symlinking it.
This implements the suggestion from Simon McVittie on https://bugzilla.gnome.org/show_bug.cgi?id=787072.
It also handles source directories not existing, which for example /lib64 won't on 32-bit systems.