Add ShellCheck's --enable=all inside scripts/

Strengthen static code analysis for shell scripts.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: John Kennedy <john.kennedy@delphix.com>
Signed-off-by: szubersk <szuberskidamian@gmail.com>
Closes #12914

scripts/paxcheck.sh

@@ -40,4 +40,4 @@ if [ x"${OUT}" != x ]; then
     echo
 fi
 
-exit $RET
+exit "$RET"