openhandle FH

    Returns FH if FH may be used as a filehandle and is open,
    or FH is a tied handle. Otherwise undef is returned.

	$fh = openhandle(*STDIN);           # \*STDIN
	$fh = openhandle(\*STDIN);          # \*STDIN
	$fh = openhandle(*NOTOPEN);         # undef
	$fh = openhandle("scalar");         # undef