Review fixes. The first-boot systemd-analyze now waits (bounded) for
startup to finish, since SSH can answer first and systemd-analyze
refuses until it has. In the boot scenario, a non-zero exit from
is-system-running (as for "degraded") no longer ends the script under
set -e before the diagnostics are saved, and a failed query for failed
units writes a marker instead of an empty file that read as "none".