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

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.