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.

See freebsd-update(8) for details.

PR: 240757, 240177, 229346
Reviewed by: manpages (bcr), sectam (emaste), yuripv
Differential Revision: https://reviews.freebsd.org/D21473