sandbox/vfs: implement proc_pid_mountinfo(5) parser

Test cases are mostly taken from util-linux. This implementation is more correct and slightly faster than the one found in github:kubernetes/utils.

Signed-off-by: Ophestra <cat@gensokyo.uk>

sandbox/vfs/mangle.go

@@ -0,0 +1,30 @@
+package vfs
+
+import "strings"
+
+func Unmangle(s string) string {
+	if !strings.ContainsRune(s, '\\') {
+		return s
+	}
+
+	v := make([]byte, len(s))
+	var (
+		j int
+		c byte
+	)
+	for i := 0; i < len(s); i++ {
+		c = s[i]
+		if c == '\\' && len(s) > i+3 &&
+			(s[i+1] == '0' || s[i+1] == '1') &&
+			(s[i+2] >= '0' && s[i+2] <= '7') &&
+			(s[i+3] >= '0' && s[i+3] <= '7') {
+			c = ((s[i+1] - '0') << 6) |
+				((s[i+2] - '0') << 3) |
+				(s[i+3] - '0')
+			i += 3
+		}
+		v[j] = c
+		j++
+	}
+	return string(v[:j])
+}