Fix singleton MPI_Abort. Singletons no longer immediately start an HNP, but only launch one when they need it for comm_spawn. So there isn't anyone to send the "abort" report to, and thus we just exit after emitting our message.

cmr=v1.7.5:reviewer=jsquyres:subject=Fix singleton MPI_Abort
add help for --with-hcoll
Added by Josh, reviewed by Mike
cmr=v1.7.5:reviewer=ompi-rm1.7