Improve management of temp files

[arno_b]

An attacker can stop latexdraw (e.g. send it SIGSTOP), see
what temp file it is using, symlink /tmp/latexdrawTmp1234567.aux to any
random file that the user has permission to write to and then latex will fill
that file with rubbish.

The standard approach for this is to use a separate directory within /tmp for
the set of temp files. The normal way of doing this in a shell script is with
the "mktemp -d" command. The start of some
code to do this properly can be found at:

       http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=4735419

as you can see from the comments on the page, it's a non-trivial thing to get
right... marking the directory as .deleteOnExit() also seems sensible to make
sure it's cleaned up should latexdraw crash prior to cleanup.