[Libguestfs] [PATCH nbdkit v3] New tmpdisk plugin.

v2 was here: https://www.redhat.com/archives/libguestfs/2020-March/msg00154.html v3: - Micro-optimize tmpdir. - Quote $disk in default command shell fragment. - Don't redirect mkfs output to /dev/null. Instead use exec </dev/null >/dev/null before the shell fragment. We may want to do this in other places where we run external shell scripts, or more generally for all plugins, but this commit does not fix this. - Improve can_multi_conn comment. - Use mkostemp if available. If not we have to use a racy mkstemp + set_cloexec instead. - I still didn't implement .zero, because the implementation (see plugins/file/file.c) is really complicated even if you remove the block device code. I guess it would be nice to isolate all this complexity into common/ at some point, which would allow us to implement efficient zero (and trim) in other file-backed plugins. - I left WIFSTOPPED, but I guess I could remove it. Are we sure it can never happen (eg. if the user is running nbdkit in the foreground and uses ^Z)? - Rerun the tests & valgrind. Rich.