Fix bug when using more than one standalone worker

The worker_temp_directory for standalone workers was set to
/tmp/worker_${USER}.standalone. If two standalone workers were running
on the machine, they could overwrite each other’s files.
The fix include the process ID ($$ in Perl) in the directory name.
Merge pull request #3 from jherrero/version/1.9

bugfix: when using more than one standalone worker make sure their /tmp directories do not clash