rio looks backwards in the line for the beginning of a filename
that needs to be completed with ^F. this change makes the
characters: =, ^, ( and { stoppers, so filename completion
will work in all these cases:
foobar=/foo/ba^F; for(i in (fo^F ba^F)){/bin/baz^F
this means completion will not work for prefixes having these
special characters in them.
thanks to burnzez for bringing it up.