Style check shell scripts

If the command "shellcheck" exists, then find all shell scripts and
run shellcheck on them.

• Use 'gcc' format with shellcheck.
• Exclude zfs-script-config.sh (which isn't really a script).

Signed-off-by: Turbo Fredriksson <turbo@bayour.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #3428