[PATCH] nbd/server: Add --selinux-label option

[Richard W.M. Jones]

https://bugzilla.redhat.com/show_bug.cgi?id=1984938

The purpose of the patch is explained in the commit message / bug.  In
the cover I want to explain a couple of design choices.

If libselinux isn't available at build time then the --selinux-label
option is still present.  It does not appear in the qemu-nbd --help
output.  If you still use it, it is ignored.  (By contrast nbdkit will
give an error if you try to use the option without having SELinux
support.  It's not clear which is better.)

We give an error if setsockcreatecon_raw fails.  In theory we could
ignore this error (warning?) and keep going.  Either SELinux would
later reject clients or it wouldn't.

Rich.