Verify ZFS Test Suite scripts executability

This change adds a make target 'testscheck' which verifies all ksh test
scripts, part of the ZFS Test Suite, have their executable bit set:
this should avoid adding new test scripts that cannot be executed by
the test-runner.py which would result in the following warning message:

   Warning: Test removed from TestGroup because it failed verification.

This also verifies both *.cfg and *.kshlib files used in the ZFS Test
Suite are not executable.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Signed-off-by: loli10K <ezomori.nozomu@gmail.com>
Closes #7131