Fix minor MPI thread memory leak / fix valgrind still-reachable warning.

cmr=v1.7.5:reviewer=brbarret:subject=Fix minor MPI thread memory leak
Minor improvements to the usnic BTL:

 1. Fix ompi_info memory leak in usnic BTL: do not allocate memory in
    the component register function, because ompi_info only calls the
    component register function and then dlclose's the component -- it
    does not call component finalize.  Instead, defer parsing the MCA
    param (and alloc'ing memory) until the component init function so
    that any allocated memory can be freed in the component close
    function.
 1. Also add a new check to ensure that we actually have some part
    numbers to check.  Add a show_help message if we don't find any
    vendor part IDs to check.
 1. Add a verbose output if usnic disqualifies itself from selection
    because THREAD_MULTIPLE was specified.

cmr=v1.7.5:reviewer=dgoodell