fn: dind SIGINT and SIGCHLD changes (#771)

1) in dind, prevent SIGINT reaching to dockerd. This kills
docker and prevents shutdown as fn server is trying to stop.
2) as init process, always reap child processes.

images/dind/preentry.sh

@@ -1,5 +1,5 @@
 #!/bin/sh
-set -e
+set -euo pipefail
 
 fsdriver=$(grep -Eh -w -m1 "overlay|aufs" /proc/filesystems | cut -f2)
 if [ $fsdriver == "overlay" ]; then
@@ -10,6 +10,8 @@ mtu=$(ip link show dev $(ip route |
                          awk '$1 == "default" { print $NF }') |
       awk '{for (i = 1; i <= NF; i++) if ($i == "mtu") print $(i+1)}')
 
+# activate job control, prevent docker process from receiving SIGINT
+set -m
 dockerd-entrypoint.sh --storage-driver=$fsdriver --mtu=$mtu &
 
 # give docker a few seconds