Linux containers on FreeBSD

This allows running Linux containers on FreeBSD and modifies the
mounts so that they represent the linux emulated filesystems, as per:
https://wiki.freebsd.org/LinuxJails

Co-authored-by: Gijs Peskens <gijs@peskens.net>, Samuel Karp <samuelkarp@users.noreply.github.com>
Signed-off-by: Artem Khramov <akhramov@pm.me>

oci/spec_opts_linux.go

@@ -203,3 +203,7 @@ func WithCDI(annotations map[string]string, cdiSpecDirs []string) SpecOpts {
 		return nil
 	}
 }
+
+func appendOSMounts(s *Spec, os string) error {
+	return nil
+}