MFC r352513, r352514, r352662:

As suggested by koobs, MFC recent freebsd-update changes to
have a consistent user experience on all supported
versions.

r352513:
freebsd-update.8: appease igor

igor follows American style guides in the belief that abbreviations i.e.
and e.g. are always followed by a comma. Make that change now so that
future updates to freebsd-update.8 do not complain about this.

r352514:
freebsd-update: make usage output consistent

Drop trailing . which appeared only on description of IDS.

r352662:
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.

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