Extend zloop.sh for automated testing

In order to debug issues encountered by ztest during automated
testing it's important that as much debugging information as
possible by dumped at the time of the failure. The following
changes extend the zloop.sh script in order to make it easier
to integrate with buildbot.

• Add the -m <maximum cores> option to zloop.sh to place a limit of the number of core dumps generated. By default, the existing behavior is maintained and no limit is set.

• Add the -l option to create a 'ztest.core.N' symlink in the current directory to the core directory. This functionality is provided primarily for buildbot which expects log files to have well known names.

• Rename 'ztest.ddt' to 'ztest.zdb' and extend it to dump additional basic information on failure for latter analysis.

Reviewed-by: Tim Chase <tim@chase2k.com>
Reviewed by: Thomas Caputi <tcaputi@datto.com>
Reviewed-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #6999