Correct flaws in arc_summary[23] and their test.

The change correctly handles BrokenPipeError and improves the
associated tests.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: John Kennedy <john.kennedy@delphix.com>
Signed-off-by: Rich Ercolani <rincebrain@gmail.com>
Closes #12037
Closes #12036

tests/zfs-tests/tests/functional/cli_user/misc/arc_summary_001_pos.ksh

@@ -48,6 +48,9 @@ else
 	set -A args  "" "-a" "-d" "-p 1"
 fi
 
+# Without this, the below checks aren't going to work the way we hope...
+set -o pipefail
+
 typeset -i i=0
 while [[ $i -lt ${#args[*]} ]]; do
 	log_must eval "arc_summary ${args[i]} > /dev/null"