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/user_fuzzer.go

@@ -0,0 +1,43 @@
+//go:build gofuzz
+// +build gofuzz
+
+package user
+
+import (
+	"io"
+	"strings"
+)
+
+func IsDivisbleBy(n int, divisibleby int) bool {
+	return (n % divisibleby) == 0
+}
+
+func FuzzUser(data []byte) int {
+	if len(data) == 0 {
+		return -1
+	}
+	if !IsDivisbleBy(len(data), 5) {
+		return -1
+	}
+
+	var divided [][]byte
+
+	chunkSize := len(data) / 5
+
+	for i := 0; i < len(data); i += chunkSize {
+		end := i + chunkSize
+
+		divided = append(divided, data[i:end])
+	}
+
+	_, _ = ParsePasswdFilter(strings.NewReader(string(divided[0])), nil)
+
+	var passwd, group io.Reader
+
+	group = strings.NewReader(string(divided[1]))
+	_, _ = GetAdditionalGroups([]string{string(divided[2])}, group)
+
+	passwd = strings.NewReader(string(divided[3]))
+	_, _ = GetExecUser(string(divided[4]), nil, passwd, group)
+	return 1
+}