Back-port the pulse-auto-update.sh half of v5 fix 0f2982ce3 to v6. After a
verified update, if Pulse was running beforehand, wait_for_service_active
polls up to 20s, tries one explicit start, and if it still does not come
back up restores the backup binary/VERSION, restarts, and returns failure
instead of leaving Pulse stopped (common on unprivileged LXC where the
installer's restart silently fails). Adds a BASH_SOURCE guard so the
script can be sourced, plus scripts/tests/test-pulse-auto-update.sh.
Note: the install.sh start_pulse STOPPED_PULSE_SERVICE guard (interactive
installer path) still needs the stop-tracking infrastructure wired into
v6's update flow; tracked as a follow-up.
672e819850