Fix for --disable-ifem
Initialize threading support properly with MPI2
Proceed anyway without MPI_THREAD_FUNNELED

Making appropriate warnings less imperative is the price we pay for
making overzealous warnings non-lethal.
Only output warnings from rank 0
Add libmesh_warning() to --enable-warnings
Use libmesh_warning() when unsure of MPI+threads

This way anyone who doesn't want users to be spammed with
usually-false-positive warnings can shut them up.