secure stream related critical section against thread cancellation.
(_newlib_flockfile_exit): Ditto.
(_newlib_sfp_lock_end): Ditto.
(_newlib_sfp_lock_start): Ditto for the list of streams.
(_newlib_sfp_lock_exit): Ditto.
(_newlib_sfp_lock_end): Ditto.
Use aforementioned macros in place of _flockfile/_funlockfile
and __sfp_lock_acquire/__sfp_lock_release throughout the code.
* libc/stdio/fclose.c: Explicitely disable and re-enable thread
cancellation. Explain why.
* libc/stdio/freopen.c: Ditto.
* libc/stdio64/freopen64.c: Ditto.
