fabrics: Use /etc/machine-id as a fallback source for system UUID

[tbzatek]

In case when system DMI data are unavailable or bogus, fall back to reading /etc/machine-id before resorting to generating a random UUID.

The machine-id file typically contains persistent and unique machine UUID stable for the OS instance.

See https://www.freedesktop.org/software/systemd/man/latest/machine-id.html

Note: while this appears to be part of systemd, the file is present on my OpenRC system, likely created by elogind.

TODO:

• The specs say the value is confidential and is supposed to be hashed before used publicly. The suggested sd_id128_get_machine_app_specific(3) function just performs SHA256 hash with an arbitrary application ID value, however given that openssl use is optional and this should work in a minimal environment, we could perhaps xor the value with some const. Not sure how useful would that be though.

[igaw]

libnvme already has a dependency to openssl, so it would be possible to use the sha algo from there, see the _tls_key functions. It wouldn't be too hard to get this working if openssl is present.

And when the library is not available just don't support /etc/machine-id? If someone is actually using the library on a small machine, it would be surprised if nvmeof is a topic.