Don't fail on setting -ve oom score when rootless

This allows non-privileged users to use containerd.
If a non root user tried to set a negative oom score adjustment,
it will fail. Containerd should not fail if running rootless.

This is part of a larger track of work integrating containerd
into Cloudfoundry's garden with support for rootless.

[#156343443]

Signed-off-by: Danail Branekov <danailster@gmail.com>

sys/env.go

@@ -0,0 +1,33 @@
+// +build !windows
+
+/*
+   Copyright The containerd Authors.
+
+   Licensed under the Apache License, Version 2.0 (the "License");
+   you may not use this file except in compliance with the License.
+   You may obtain a copy of the License at
+
+       http://www.apache.org/licenses/LICENSE-2.0
+
+   Unless required by applicable law or agreed to in writing, software
+   distributed under the License is distributed on an "AS IS" BASIS,
+   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+   See the License for the specific language governing permissions and
+   limitations under the License.
+*/
+
+package sys
+
+import "golang.org/x/sys/unix"
+
+// RunningPrivileged returns true if the effective user ID of the
+// calling process is 0
+func RunningPrivileged() bool {
+	return unix.Geteuid() == 0
+}
+
+// RunningUnprivileged returns true if the effective user ID of the
+// calling process is not 0
+func RunningUnprivileged() bool {
+	return !RunningPrivileged()
+}