MF stable/12 r352759,r352771

MF stable/12 r352759,r352771

Approved by:	re (gjb)

r352759:
freebsd-update.8: Style fixes, document new features.
freebsd-update:   Make usage output consistent.
freebsd-update:   Add `updatesready' and `showconfig' commands
freebsd-update:   Change exit code of `freebsd-update install' to 2
                  in case there are no pending updates and there wasn't
                  a fetch phase in the same invocation.

r352771:
Add mergeinfo missing in r352759

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