We need mutex protection in PerlIOStdio_close() for the duration of

We need mutex protection in PerlIOStdio_close() for the duration of
holding our true love file handle open, to stop anything else
temporarily using it for a quick dup() fling, and then closing the
file handle underneath us.
I suspect that the lack of this protection was the cause of the threads
free.t and blocks.t failures on OS X on 5.8.x, where usefaststdio is
the default, and PerlIO is unable to "invalidate" the FILE *.

p4raw-id: //depot/perl@33492