[Mono/Linux] Make sure temp files of different users can't clash

On Mono/Linux, GetTempPath() returns /tmp. This creates a problem for
multiple users of HFM (concurrent or not) as temp HTML/XML files may clash.

Mitigate the issue by having HFM use /tmp/hfm-<user> location for temporary
HTML/XML files.

In the long run we should probably incorporate .NET's equivalent of
mkstemp() or mkdtemp().