losmoketest.py: fix to work with LO-3.5 daily build and improved

+ use the version 3.5 instead of 3.4
+ use /opt/lo-dev instead of /opt/libreoffice3.4
+ new options:
    + -d/--download just download the last build
    + -s/--headless use the headless mode when running the smoke test
    + -l/--loop do download/install/smoketest in a loop; do it jsut once
      by default
+ uninstall the old version when -i/--install option is used
+ mention PASSED/FAILED in the test log
+ restore compat symlinks to prebuilt binaries when they do not exits
+ complain when build.cfg is missing