ABC-22: Plugin sandboxing for linux/amd64 (#8068)

* plugin sandboxing

* remove unused type

* better symlink handling, better remounting, better test, whitespace
fixes, and comment on the remounting

* fix test compile error

* big simplification for getting mount flags

* mask statfs flags to the ones we're interested in

plugin/rpcplugin/sandbox/seccomp_linux_other.go

@@ -0,0 +1,10 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See License.txt for license information.
+
+// +build linux,!amd64
+
+package sandbox
+
+const NATIVE_AUDIT_ARCH = 0
+
+var AllowedSyscalls []SeccompSyscall