supplementary group support

C version of virtiofsd has an issue open where client expects supplementary group info will be used to allow file creation in parent dir but virtiofsd/fuse can't handle that. I think rust virtiofsd has same issue. So opening this report.

Here is the qemu issue.

qemu-project/qemu#368

Here is a email thread to a discussion about this issue.

https://listman.redhat.com/archives/virtio-fs/2021-June/msg00001.html

Recently a JIRA issue has been opened by KATA folks who are trying to run mysqldb inside kata container with virtiofs as storage.

https://issues.redhat.com/browse/KATA-1776

I guess it will be nice if we can figure a way out to fix this.