This checkin mainly concerns our internal info keys that are registering
for callbacks via opal_infosubscribe_subscribe(). Those keys need to have
an extra __IN_<key>/val stored to preserve their pre-callback value. So
that means our internal keys are limited to 5 chars shorter than the usual
key length limit.
The code previously would have been silently inactive if a large key happened
to come in, now it warns and also uses snprintf() to avoid compiler warnings.
I'm also making the top-level MPI_Info_set warn if the user uses our reserved
"__IN_" prefix. I had wanted the feature to be more invisible than that, but
it would require a more sophisticated approach to change that.
Signed-off-by: Mark Allen <markalle@us.ibm.com>