nbd/server: Add --selinux-label option

Under SELinux, Unix domain sockets have two labels.  One is on the
disk and can be set with commands such as chcon(1).  There is a
different label stored in memory (called the process label).  This can
only be set by the process creating the socket.  When using SELinux +
SVirt and wanting qemu to be able to connect to a qemu-nbd instance,
you must set both labels correctly first.

For qemu-nbd the options to set the second label are awkward.  You can
create the socket in a wrapper program and then exec into qemu-nbd.
Or you could try something with LD_PRELOAD.

This commit adds the ability to set the label straightforwardly on the
command line, via the new --selinux-label flag.  (The name of the flag
is the same as the equivalent nbdkit option.)

A worked example showing how to use the new option can be found in
this bug: https://bugzilla.redhat.com/show_bug.cgi?id=1984938

Fixes: https://bugzilla.redhat.com/show_bug.cgi?id=1984938
Signed-off-by: Richard W.M. Jones <rjones@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>

[eblake: rebase to configure changes, reject --selinux-label if it is
not compiled in or not used on a Unix socket]
Note that we may relax some of these restrictions at a later date,
such as making it possible to label a TCP socket, although it may be
smarter to do so as a generic QMP action rather than more one-off
command lines in qemu-nbd.
Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <20211115202944.615966-1-eblake@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
[eblake: adjust meson output as suggested by thuth]
Signed-off-by: Eric Blake <eblake@redhat.com>

tests/docker/dockerfiles/opensuse-leap.docker

@@ -55,6 +55,7 @@ ENV PACKAGES \
     libpulse-devel \
     librbd-devel \
     libseccomp-devel \
+    libselinux-devel \
     libspice-server-devel \
     libssh-devel \
     libtasn1-devel \