Recovery could not tell a daemon restart from a host reboot: both
looked like "some sessions were not in tmux list-sessions", so the
tmux probe ran unconditionally and the logs said nothing useful when
it was actually a reboot.
Record /proc/sys/kernel/random/boot_id in the state directory on
every recover(). When it differs from the stored value, every session
from the previous boot is provably dead, so the tmux probe is skipped
and the second recovery pass registers them dormant directly, logging
"host rebooted, sessions dormant" with the count. A missing stored
value or an unreadable boot id (non-Linux, restricted /proc) falls
back to today's probe-based recovery.
The boot id source is injected via a BootIdSource trait so tests can
simulate a changed or unreadable boot id without rebooting the
machine.
Refs #164
Miguel Palhas
d548d0b24e