Allow to limit zed's syslog chattiness

Some usage patterns like send/recv of replication streams can
produce a large number of events. In such a case, the current
all-syslog.sh zedlet will hold up to its name, and flood the
logs with mostly redundant information. Two mitigate this
situation, this changeset introduces to new variables
ZED_SYSLOG_SUBCLASS_INCLUDE and ZED_SYSLOG_SUBCLASS_EXCLUDE
to zed.rc that give more control over which event classes end
up in the syslog.

Reviewed-by: loli10K <ezomori.nozomu@gmail.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Signed-off-by: Tony Hutter <hutter2@llnl.gov>
Signed-off-by: Daniel Kobras <d.kobras@science-computing.de>
Closes #6886
Closes #7260

tests/zfs-tests/include/libtest.shlib

@@ -3398,6 +3398,21 @@ function wait_scrubbed
 	return 1
 }
 
+# Backup the zed.rc in our test directory so that we can edit it for our test.
+#
+# Returns: Backup file name.  You will need to pass this to zed_rc_restore().
+function zed_rc_backup
+{
+	zedrc_backup="$(mktemp)"
+	cp $ZEDLET_DIR/zed.rc $zedrc_backup
+	echo $zedrc_backup
+}
+
+function zed_rc_restore
+{
+	mv $1 $ZEDLET_DIR/zed.rc
+}
+
 #
 # Setup custom environment for the ZED.
 #
@@ -3536,6 +3551,23 @@ function zed_events_drain
        done
 }
 
+# Set a variable in zed.rc to something, un-commenting it in the process.
+#
+# $1 variable
+# $2 value
+function zed_rc_set
+{
+	var="$1"
+	val="$2"
+	# Remove the line
+	cmd="'/$var/d'"
+	eval sed -i $cmd $ZEDLET_DIR/zed.rc
+
+	# Add it at the end
+	echo "$var=$val" >> $ZEDLET_DIR/zed.rc
+}
+
+
 #
 # Check is provided device is being active used as a swap device.
 #