_synthesize(): Helper function: when the users passes a specific

_synthesize():  Helper function:  when the users passes a specific
                value for the 'using' parameter of the get() function
                or the BROWSER environment variable, if the thing
                passed in is a path (as seems to be the case with KDE)
                instead of a short name, examine the available
                controllers to see if we can synthesize one based on a
                pre-registered controller that shares the same base
                name.

get():  If the user specifies a browser we don't know about, use
        _synthesize() to attempt to create a usable controller.

Some small adjustments were needed in some of the browser classes to
support this.