main: Dump debug log to plymouth-shutdown-debug.log on shutdown/reboot

main: Dump debug log to plymouth-shutdown-debug.log on shutdown/reboot

When working on plymouth I always have "plymouth.debug=stream:/dev/null"
on the kernel commandline. This enables tracing without logging anything
to the console and causes the entire trace to be logged to
/var/log/plymouth-debug.log when plymouth quits.

This is very useful for debugging (non crash) issues with plymouth at boot.
With the recent "main: Cleanly quit on SIGTERM" change plymouth will now
also write a trace log to /var/log/plymouth-debug.log on shutdown/reboot,
but this will be overwritten again on boot by the boot log.

This commit changes the default debug_buffer_path value from:
/var/log/plymouth-debug.log to /var/log/plymouth-shutdown-debug.log
when in shutdown or reboot mode so that it does not get overwritten
by the debug-log written at boot.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>