[Libguestfs] [PATCH nbdkit 1/2] file: Add an internal "mode"

[Richard W.M. Jones]

This is upstream in 5 commits 5764fa6f4..dd28b0054:

https://gitlab.com/nbdkit/nbdkit/-/commit/2620d95a4585204f0db0d55c0d41276d11970436
https://gitlab.com/nbdkit/nbdkit/-/commit/185e7d4010b353f36b5ca5d47467a770c530e58c
https://gitlab.com/nbdkit/nbdkit/-/commit/17655b1ca67caa454e7a4ac83bc8c052b79a692d
https://gitlab.com/nbdkit/nbdkit/-/commit/e7fdffde8142fb083625678b7a55455751185502
https://gitlab.com/nbdkit/nbdkit/-/commit/dd28b005430d020ccd1825437937c317332d3007

This also includes dirfd functionality and a rather complicated test
for that.  Turns out that bash refuses to open a directory as a file
descriptor :-(

Rich.

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