API functions do not know if they are being called from a web page, or a
web-service. They need a single function to call to correctly apply
filters, strip tags etc that knows the correct way to do this based on
the way it was called (e.g. webservices allow parameters for skipping
filters etc).

So here we are fixing the default for the filter argument used in
external_format_text and adding a matching external_format_string.