freebsd-update: Don't provide copiable commands in output

Previously, freebsd-update provided ready-to-go commands for copying and
pasting into the terminal. This causes problems as soon as options are
used and not supplied again by the user, e.g., '-b' or '-d'.
Stop making them copiable and force the user to construct a valid command
line by himself to avoid failures.

PR: 276102
Approved by: jrm (mentor), emaste
MFC after: 1 month
Differential Revision: https://reviews.freebsd.org/D43700