appliance: mount also /dev/pts

Tools could require the use of pseudo-terminals, so make sure we have
/dev/pts available in the appliance.  The "command" API already
bind-mounts it when running commands, so this is the only bit needed.

appliance/init

@@ -72,6 +72,8 @@ fi
 
 # devtmpfs is required since udev 176
 mount -t devtmpfs /dev /dev
+mkdir -p /dev/pts
+mount -t devpts /dev/pts /dev/pts
 
 if [[ $cmdline == *selinux=1* ]]; then
   mount -t selinuxfs none /sys/fs/selinux