petscnagupgrade.py: use ${PYTHON} and make compatible with Python-3
petsc_stageLog: fix for --with-log=0
Bug fix for MatLoad_MPIBAIJ() buffer size for rank zero read was wrong size; simplified code as well MatLoad_MPIAIJ() also
This fix can also be ported back to 3.3
VecView_MPI_HDF5: remove unused variable
Bug fix for MatLoad_MPIBAIJ() buffer size for rank zero read was wrong size; simplified code as well MatLoad_MPIAIJ() also

Cherry-pick 8de993d478c580efae724da89164c5c6bd81be18

Conflicts:
	src/mat/impls/aij/mpi/mpiaij.c
Merge branch 'maint'

Conflicts:
	conf/rules
	include/petsclog.h
Merge branch 'master' into next
Merge branch 'barry/matload-mpiaj-bugfix-maint' into next