fi_tostr fixes

Two fixes for fi_str:

1. Don't use ```__thread``` for TSD; it's a GCC extension.  So just explicitly declare that fi_tostr() is not thread safe.
1. Ensure that fi_tostr() does not overflow its buffer.
Sorry -- hit the button too soon.  I don't know if BUFSIZ is POSIX or what, but it's been in every stdio.h that I've ever ported Open MPI to.  On RHEL 6, it's 8K.