Debian squeeze/wheezy startup-script + new alternative diff update script

Hi roland,

On our api.openstreetmap.fr instance (running debian squeeze) I felt the need for a different (and somewhat easier) way to manage dispatcher(s) and diff update process.

This pull requests includes the /debian/overpass SysV startup script for dispatcher(s) and diff update with a way to record one's own $DB_DIR $EXEC_DIR diff sources in a debian/ubuntu specific place (/etc/default/overpass) to avoid expressing them as command line arguments at every restart or after software updates.

This pull request also adds src/bin/fetch_osc_and_apply.sh
That script is kind of a merge of fetch_osc.sh & apply_osc_to_db.sh in one. For people who don't want to maintain a local copy of all diffs (and handle the clean up).
This was convenient for me to just download and apply (using update_database instead of update_from_dir)

Feel free to merge it in your tree if you think there might be intereset, it shouldn't change usual behaviour.