Fix fflush issues.

Fix fflush issues.
* libc/stdio/fflush.c (_fflush_r): New function.
(fflush): Fix reentrancy and large offset behavior.
* libc/include/stdio.h (_fflush_r): Add prototype.
* libc/stdio/fclose.c (_fclose_r): All fflush callers changed.
* libc/stdio/freopen.c (_freopen_r): Likewise.
* libc/stdio/fseek.c (_fseek_r): Likewise.
* libc/stdio/ftell.c (_ftell_r): Likewise.
* libc/stdio/fvwrite.c (__sfvwrite_r): Likewise.
* libc/stdio/refill.c (__srefill_r): Likewise.
* libc/stdio/setvbuf.c (setvbuf): Likewise.
* libc/stdio/ungetc.c (_ungetc_r): Likewise.
* libc/stdio/vfprintf.c (__sbprintf): Likewise.
* libc/stdio/wbuf.c (__swbuf_r): Likewise.
* libc/stdio64/freopen64.c (_freopen64_r): Likewise.
* libc/stdio64/fseeko64.c (_fseeko64_r): Likewise.  Defer to
32-bit version if not large file.
* libc/stdio64/ftello64.c (_ftello64_r): Likewise.
* libc/stdio64/tmpfile64.c (_tmpfile64_r): Avoid compile warning.