Improve bash quoting, and other similar best practices

This patch updates the shell scripts with some of the common best
practices, which should make them more resilient to unusual failures and
unexpected environments (in particular, directories with spaces).

Most of these were identified by shellcheck.

test/t-16-spf/run.sh

@@ -5,7 +5,7 @@
 # main gaps.
 
 set -e
-. $(dirname ${0})/../util/lib.sh
+. "$(dirname "$0")/../util/lib.sh"
 
 init
 check_hostaliases
@@ -47,7 +47,7 @@ function launch_minidns() {
 		kill $MINIDNS
 		wait $MINIDNS || true
 	fi
-	cp $1 .zones
+	cp "$1" .zones
 	minidns_bg --addr=":9053" -zones=.zones >> .minidns.log 2>&1
 	wait_until_ready 9053
 }