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_test.go

@@ -0,0 +1,27 @@
+package vfs_test
+
+import (
+	"testing"
+
+	"git.gensokyo.uk/security/fortify/sandbox/vfs"
+)
+
+func TestUnmangle(t *testing.T) {
+	testCases := []struct {
+		want   string
+		sample string
+	}{
+		{`\, `, `\134\054\040`},
+		{`(10) source -- maybe empty string`, `(10)\040source\040--\040maybe empty string`},
+	}
+
+	for _, tc := range testCases {
+		t.Run(tc.want, func(t *testing.T) {
+			got := vfs.Unmangle(tc.sample)
+			if got != tc.want {
+				t.Errorf("Unmangle: %q, want %q",
+					got, tc.want)
+			}
+		})
+	}
+}