zfskeys: allow prompt to entered during boot up

If the ZFS key is setup in prompt mode, use zfs to prompt to load the key
during boot to unlock it.

Reviewed by:	jhb
Differential Revision:	https://reviews.freebsd.org/D36081

libexec/rc/rc.d/zfskeys

@@ -45,7 +45,15 @@ unlock_fs()
     local kl="$2"
     local k="${kl##file://}"
 
-    if [ "$k" ] && [ -f "$k" ] && [ -s "$k" ] && [ -r "$k" ]; then
+    if [ "$kl" == "prompt" ]
+    then
+        echo "Key prompt for $fs."
+        if zfs load-key -L "$kl" "$fs" < /dev/tty > /dev/tty 2>/dev/tty ; then
+	    echo "Key loaded for $fs."
+        else
+	    echo "Key failed to load for $fs."
+        fi
+    elif [ "$k" ] && [ -f "$k" ] && [ -s "$k" ] && [ -r "$k" ]; then
         if [ "$(zfs get -Ho value keystatus "$fs")" = 'available' ]; then
             echo "Key already loaded for $fs."
         elif keytest=$(zfs load-key -n -L "$kl" "$fs" 2>&1); then