Make compiler.properties fall back to prefixed

Previously, if we wanted to override the shell.prompt property, we had
to modify compiler.properties in the jar. This change lets us do the
following, instead:

    scala -Dscala.shell.prompt="$(echo -e "\npuffnfresh> ")"

All properties previously loaded from compiler.properties now fall back
to "scala." in the system properties when not found.
Could you please document the fallback mechanism here?