A variant of the M.2 RAID enablement kit does not manifest with nvme
driver. Address this by allowing 'nvm' subsystype. to allow blank driver.
Also, to be on the safe side, have self.driver always be a string,
so it can be 'falsey' but still work as a string.
If syncfiles fails, keep it retrying.
Also, slow down sync checking to avoid hammering the system.
Further, randomized delay to spread highly synchronized requestors.
Block attempts to do multiple concurrent syncfile runs.
Some versions start manifesting nvme devnames with 'c', which
are to be used to interact with multipath to have raw devices
backing a traditional nvme device.
With significant firstboot output, there was a tendency
for tail to be killed before it relayed all the content.
Change to run the firstboot in a subshell in the background,
and have tail explicitly run until that subshell naturally
exits and then tail will cleanly exit
