Remove useless "invalid SSH_HOSTS" warning
- Do not call 'fpart' through 'time' anymore
  (useless since we already record start and stop times independently)
- Introduce VERBOSE mode and modify echo_log() accordingly
Do not hardcode paths to binaries
- Do not ask for FPART_JOBNAME anymore ; it is now auto-generated
  (using a sha1 of dst_dir/ as well as the process PID and a timestamp)
- Relocate JOBS_QUEUEDIR and JOBS_WORKDIR to /tmp by default
- Introduce FPART_SHDIR, used as a basis for FPART_OUTPARTDIR and FPART_LOGDIR
- Do not put PID in log file names anymore (already present in parent log dir)
- Cleanup final status/mail message
- Create dst_dir/ automatically and avoid split-brain situations when using SSH
- Rephrase directory-related error messages
Rename a few variables