Revert "Revert "Switch to running custard as a custard user rather than root""

This reverts commit 8c9341bea6c8305f36bc74f6c086cc13d57963e2.
git-daemon should run as custard:custard
/opt/tools should be owned by custard:custard
www-data user should belong to custard group
stop/start git-daemon rather than restarting it
chown various directories to custard
Close #11