freebsd-update: Add `updatesready' and `showconfig' commands
`freebsd-update updatesready' can be used to check if there are any pending
fetched updates that can be installed.
`freebsd-update showconfig' writes freebsd-update's configuration to
stdout.
This also changes the exit code of `freebsd-update install' to 2 in case
there are no updates pending to be installed and there wasn't a fetch phase
in the same invocation. This allows scripts to tell apart these error
conditions without breaking existing jail managers.