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