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.

Closes: #4
Closes: #89
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00:36:43