Add Makefile command to run checkbashisms on all /bin/sh scripts

Based on the shellcheck make target, add a target which checks
for violations of POSIX standards for shell scripts

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Gabriel A. Devenyi <gdevenyi@gmail.com>
Closes #10513