filtered-path endpoint (#6132)

* First pass at filtered-path endpoint.  It seems to be working, but there are tests missing, and possibly some optimization to handle large key sets.

* Vendor go-cmp.

* Fix incomplete vendoring of go-cmp.

* Improve test coverage.  Fix bug whereby access to a subtree named X would expose existence of a the key named X at the same level.

* Add benchmarks, which showed that hasNonDenyCapability would be "expensive" to call for every member of a large folder.  Made a couple of minor tweaks so that now it can be done without allocations.

* Comment cleanup.

* Review requested changes: rename some funcs, use routeCommon instead of
querying storage directly.

* Keep the same endpoint for now, but move it from a LIST to a POST and allow multiple paths to be queried in one operation.

* Modify test to pass multiple paths in at once.

* Add endpoint to default policy.

* Move endpoint to /sys/access/filtered-path.

vendor/github.com/google/go-cmp/cmp/internal/function/func.go

@@ -0,0 +1,49 @@
+// Copyright 2017, The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE.md file.
+
+// Package function identifies function types.
+package function
+
+import "reflect"
+
+type funcType int
+
+const (
+	_ funcType = iota
+
+	ttbFunc // func(T, T) bool
+	tibFunc // func(T, I) bool
+	trFunc  // func(T) R
+
+	Equal           = ttbFunc // func(T, T) bool
+	EqualAssignable = tibFunc // func(T, I) bool; encapsulates func(T, T) bool
+	Transformer     = trFunc  // func(T) R
+	ValueFilter     = ttbFunc // func(T, T) bool
+	Less            = ttbFunc // func(T, T) bool
+)
+
+var boolType = reflect.TypeOf(true)
+
+// IsType reports whether the reflect.Type is of the specified function type.
+func IsType(t reflect.Type, ft funcType) bool {
+	if t == nil || t.Kind() != reflect.Func || t.IsVariadic() {
+		return false
+	}
+	ni, no := t.NumIn(), t.NumOut()
+	switch ft {
+	case ttbFunc: // func(T, T) bool
+		if ni == 2 && no == 1 && t.In(0) == t.In(1) && t.Out(0) == boolType {
+			return true
+		}
+	case tibFunc: // func(T, I) bool
+		if ni == 2 && no == 1 && t.In(0).AssignableTo(t.In(1)) && t.Out(0) == boolType {
+			return true
+		}
+	case trFunc: // func(T) R
+		if ni == 1 && no == 1 {
+			return true
+		}
+	}
+	return false
+}