ensure non-negative counts in MPIC_ funcs

When no less than 4 tasks in the same group call MPI_Scatter with
recvcount less than sendcount, non-root tasks will pass a negative count
to MPIC_Send_ft, which won't always be caught by the device.  This adds
a bit of extra error checking to help catch this case.

Based on a patch written by Qi QC Zhang <keirazhang@cn.ibm.com>,
augmented by Dave Goodell <goodell@mcs.anl.gov>.
release.pl: more updates for git

Partially reviewed by balaji@.
autogen.sh: just copy "externals"

The interaction with git was unnecessarily conservative and caused
problems for both the release.pl script and our homebrew formula.

Reviewed-by: balaji