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.
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+
+   Copyright [yyyy] [name of copyright owner]
 
    Licensed under the Apache License, Version 2.0 (the "License");
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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:
-
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-For more information, please see http://www.bis.doc.gov
-
-See also http://www.apache.org/dev/crypto.html and/or seek legal counsel.