[Virtio-fs] [virtiofsd] MR opened: Set signal handlers, required to be an "init" process in a PID namespace

[virtiofs-bot at sinrega.org]

The first process created in a new PID namespace (pid_namespaces(7))
is the "init" process, for that namespace. Only signals for which the
"init" process has established a  signal handler can be sent to it.
To run  virtiofsd with '--sandbox namespace', we need to set handlers
to end the child process if the parent dies. Also, in case the
PID namespace is set by an external tool. Fixes #23
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https://gitlab.com/virtio-fs/virtiofsd/-/merge_requests/75