ompi_mpi_abort: fix corner cases, simplify logic

I recently found a case where ompi_mpi_abort() segv's:

{{{
$ mpirun --mca btl non_existent_btl_name ...
}}}

In this case, the BML init fails because we have no paths to any
peers.  It calls ompi_mpi_abort(), but this is before ompi_comm_self
has been setup.  ompi_mpi_abort() assumes that if the comm parameter
is != NULL, it can be used.  But since we aborted so early in
MPI_INIT, that's a false assumption.

(note that this isn't happening on v1.8 because the check for
INIT/FINALIZE in ompi_mpi_abort() is a little different.  Hence: this
is a trunk issue -- at least for now)

When fixing this problem, I noticed a few other problems in ompi_mpi_abort():

* the group access was incorrect (it didn't use accessor functions)
* it wasn't clear that ORTE's ompi_rte_abort_peers() returns
  NOT_IMPLEMENTED and falls through down to ompi_rte_abort()
* the check for my proc in the communicator was a little more
  complicated than necessary
* the logic for checking for aborts early in MPI_INIT wasn't right
* some comments were stale
* the hostname output in error messages would be NULL if MPI_FINALIZE
  had been invoked
* it was possible to abort, but still exit with a 0 status

This commit fixes all of the above problems, and makes the logic a
little more straightforward.  Thanks to Ralph Castain and George
Bosilca for the assists with this patch.