etcd: Connection options (socket file, ipv6)

- Allow unix domain socket to be used as client url
- Using ::1 as clienturl should not create default local ipv4 listener
- Add shell tests

test/shell/ipv6-localhost.sh

@@ -0,0 +1,29 @@
+#!/bin/bash
+
+set -o errexit
+set -o pipefail
+
+if ! ifconfig lo | grep 'inet6 ::1' >/dev/null; then
+	echo "No IPv6, skipping test"
+	exit 0
+fi
+
+. "$(dirname "$0")/../util.sh"
+
+tmpdir="$($mktemp --tmpdir -d tmp.XXX)"
+
+# run empty graph listing only to IPv6 addresses
+"$MGMT" run --client-urls "http://[::1]:2379" --server-urls "http://[::1]:2380" --tmp-prefix empty &
+pid=$!
+
+# kill server on error/exit
+trap 'pkill -9 mgmt' EXIT
+
+# give mgmt a little time to startup
+sleep 10
+
+# mgmt configured for ipv6 only should not listen on any IPv4 ports
+lsof -Pn -p "$pid" -a -i | grep '127.0.0.1' && false
+
+# instead it should listen on IPv6
+lsof -Pn -p "$pid" -a -i | grep '::1' || false