Changed fclose behavior when custom streams were enabled. Previously,

Changed fclose behavior when custom streams were enabled.  Previously,
the cookie pointer was set to NULL as a debugging aid.  However,
some of the perl 5.8 test rely on being able to close stderr and
still try writing to it.  So now, the cookie pointer and handler
function pointers are set to that it is a "normal" file with a
file descriptor of -1.

Note: The cookie pointer is reset to NULL if the FILE struct is
free'd by fclose.