[Libguestfs] [PATCH nbdkit v3] tmpdisk: Generalize the tmpdisk plugin.

[Richard W.M. Jones]

v2 was here:
https://www.redhat.com/archives/libguestfs/2020-April/msg00075.html

In v3:

- Add [VAR=VALUE ...] to manual.

- Various minor improvements to the manual.

- Work (at least, in theory - not tested) with block devices or
  symlinks.  I didn't document this because it's hard to ensure these
  files or block devices would be cleaned up, so here be dragons.

- Remove O_NOCTTY.

- Store the vars in order (instead of building the list in reverse).

- Add a new test of the command feature and various corner cases.

Rich.