Prevent system from sleeping while storage is attached via USB

A generic title for this issue. For starters:

### Suspend/resume versus storage on USB

https://lists.freebsd.org/pipermail/freebsd-current/2020-September/077010.html

>>> … It looks like a problem with USB resume, that no devices are recognized, until you re-plug them …

https://lists.freebsd.org/pipermail/freebsd-current/2020-November/077681.html

>> …
>> If I export the pool before suspend, no problem at resume time.
>> …
>
> Please
>
> Is there any way to avoid the need to export before suspending the computer?
>
> Might there be a future improvement to support for USB?
>
> Thanks

∴

* https://github.com/helloSystem/hello/issues/72
* ensure that helloSystem will not sleep (either on demand, or to save energy) if doing so is likely to have a **significantly adverse** effect on a mounted file system.

#### Degrees of adversity

Where the mounted file system is (for example) [FAT](https://en.wikipedia.org/wiki/File_Allocation_Table) 32 or NTFS: adversity may be negligible.

With OpenZFS on FreeBSD: if there's a single-device pool (i.e. USB flash/thumb drive or mobile HDD/SSHD/SSD) then there's near-certainty that the pool will require _at least_ a scrub, which might be frustratingly time-consuming (and I should not recommend writing to its file system(s) during the scrub). https://www.freebsd.org/cgi/man.cgi?query=zpool&sektion=8

… and so on. Not intended to be comprehensive.