Add zloop.sh test script

Add Chris Williamson's "new" zloop script so that it may be
intergated with ZoLs automated testing.  The original script may
be found in the openzfs-build repository on Github.

Minor modifications were made to the script so it can be run
directly from the ZoL source tree or from installed packages.

Additionally it was updated to use gdb instead of mdb to
extact debugging information from a core dump.

References:
  https://github.com/openzfs/openzfs-build/commit/7fb5d8b
  https://github.com/openzfs/openzfs-build/blob/master/ansible/roles/openzfs-jenkins-slave/files/usr/local/zloop.sh

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #4441

scripts/Makefile.am

@@ -10,6 +10,7 @@ dist_pkgdata_SCRIPTS = \
 	$(top_srcdir)/scripts/zimport.sh \
 	$(top_srcdir)/scripts/zfs.sh \
 	$(top_srcdir)/scripts/zfs-tests.sh \
+	$(top_srcdir)/scripts/zloop.sh \
 	$(top_srcdir)/scripts/zpool-create.sh \
 	$(top_srcdir)/scripts/zpios.sh \
 	$(top_srcdir)/scripts/zpios-sanity.sh \