Improve rcatline bad fh warnings

This is not helpful:

append I/O operator() on closed filehandle STDIN at - line 3.
append I/O operator() on closed filehandle STDIN at - line 4.
	(Are you trying to call append I/O operator() on dirhandle STDIN?)

That .=<> compiles to a separate op from <> is an implementation
detail, so show ‘readline()’ as we do with <>.