remove github.com/opencontainers/runc dependency

This migrates uses of github.com/opencontainers/runc/libcontainer/user
to the new github.com/moby/sys/user module, which was extracted from
runc at commit [opencontainers/runc@a3a0ec48c4].

This is the initial release of the module, which is a straight copy, but
some changes may be made in the next release (such as fixing camel-casing
in some fields and functions (Uid -> UID).

[opencontainers/runc@a3a0ec48c4]: a3a0ec48c4

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

vendor/github.com/moby/sys/user/LICENSE

@@ -176,7 +176,18 @@
 
    END OF TERMS AND CONDITIONS
 
-   Copyright 2014 Docker, Inc.
+   APPENDIX: How to apply the Apache License to your work.
+
+      To apply the Apache License to your work, attach the following
+      boilerplate notice, with the fields enclosed by brackets "[]"
+      replaced with your own identifying information. (Don't include
+      the brackets!)  The text should be enclosed in the appropriate
+      comment syntax for the file format. We also recommend that a
+      file or class name and description of purpose be included on the
+      same "printed page" as the copyright notice for easier
+      identification within third-party archives.
+
+   Copyright [yyyy] [name of copyright owner]
 
    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.

vendor/github.com/opencontainers/runc/NOTICE

@@ -1,17 +0,0 @@
-runc
-
-Copyright 2012-2015 Docker, Inc.
-
-This product includes software developed at Docker, Inc. (http://www.docker.com).
-
-The following is courtesy of our legal counsel:
-
-
-Use and transfer of Docker may be subject to certain restrictions by the
-United States and other governments.  
-It is your responsibility to ensure that your use and/or transfer does not
-violate applicable laws. 
-
-For more information, please see http://www.bis.doc.gov
-
-See also http://www.apache.org/dev/crypto.html and/or seek legal counsel.

vendor/modules.txt

@@ -329,6 +329,9 @@ github.com/moby/sys/signal
 # github.com/moby/sys/symlink v0.2.0
 ## explicit; go 1.16
 github.com/moby/sys/symlink
+# github.com/moby/sys/user v0.1.0
+## explicit; go 1.17
+github.com/moby/sys/user
 # github.com/modern-go/concurrent v0.0.0-20180306012644-bacd9c7ef1dd
 ## explicit
 github.com/modern-go/concurrent
@@ -344,9 +347,6 @@ github.com/opencontainers/go-digest/digestset
 github.com/opencontainers/image-spec/identity
 github.com/opencontainers/image-spec/specs-go
 github.com/opencontainers/image-spec/specs-go/v1
-# github.com/opencontainers/runc v1.1.9
-## explicit; go 1.17
-github.com/opencontainers/runc/libcontainer/user
 # github.com/opencontainers/runtime-spec v1.1.1-0.20230823135140-4fec88fd00a4
 ## explicit
 github.com/opencontainers/runtime-spec/specs-go