fi_tostr: do not use thread-specific data

Take inspiration from strerror(): just declare that fi_tostr() is not
thread safe.  Do not use __thread, because it is a GCC extension (and
it's not worth using pthread function calls to make portable thread
safe data).

Signed-off-by: Jeff Squyres <jsquyres@cisco.com>
fi_tostr: ensure that fi_tostr does not overflow the buffer

Signed-off-by: Jeff Squyres <jsquyres@cisco.com>
Merge pull request #402 from jsquyres/pr/fi-tostr-tsd

fi_tostr fixes