So far the data directory constant has pointed to the source
directory to be usable with different subdirectories. Now only
the data subdirectory itself is being used with the constant,
so point to the directory directly.
This replaces {data_dir}/settings throughout the code, so that
the configuration may be placed somewhere else in the directory
structure (e.g. in /etc).
The PHP scripts need to know the position of the nominatim
tool in order to call it. This is handed in as environment
variable, so it can be set by the Python script.
