btl/ugni: add a request_progress_thread mca param

Replace temporary environment variables with a MCA
parameter for the ugni btl.  A user wishing to
use the ugni btl async. progress thread needs to
set the request_progress_thread param to true.
For example, using env. variable format:

export OMPI_MCA_btl_ugni_request_progress_thread=1
btl/ugni: improve error handling

Improve error handling when pthread functions return errors.
Remove stale debug code.