tailfs: initial implementation

Add a WebDAV-based folder sharing mechanism that is exposed to local clients at
100.100.100.100:8080 and to remote peers via a new peerapi endpoint at
/v0/tailfs.

Add the ability to manage folder sharing via the new 'share' CLI sub-command.

Updates tailscale/corp#16827

Signed-off-by: Percy Wegmann <percy@tailscale.com>

tailfs/shared/pathutil_test.go

@@ -0,0 +1,57 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+package shared
+
+import (
+	"reflect"
+	"testing"
+)
+
+func TestCleanAndSplit(t *testing.T) {
+	tests := []struct {
+		path string
+		want []string
+	}{
+		{"", []string{""}},
+		{"/", []string{""}},
+		{"//", []string{""}},
+		{"a", []string{"a"}},
+		{"/a", []string{"a"}},
+		{"a/", []string{"a"}},
+		{"/a/", []string{"a"}},
+		{"a/b", []string{"a", "b"}},
+		{"/a/b", []string{"a", "b"}},
+		{"a/b/", []string{"a", "b"}},
+		{"/a/b/", []string{"a", "b"}},
+		{"/a/../b", []string{"b"}},
+	}
+	for _, tt := range tests {
+		t.Run(tt.path, func(t *testing.T) {
+			if got := CleanAndSplit(tt.path); !reflect.DeepEqual(tt.want, got) {
+				t.Errorf("CleanAndSplit(%q) = %v; want %v", tt.path, got, tt.want)
+			}
+		})
+	}
+}
+
+func TestJoin(t *testing.T) {
+	tests := []struct {
+		parts []string
+		want  string
+	}{
+		{[]string{""}, "/"},
+		{[]string{"a"}, "/a"},
+		{[]string{"/a"}, "/a"},
+		{[]string{"/a/"}, "/a"},
+		{[]string{"/a/", "/b/"}, "/a/b"},
+		{[]string{"/a/../b", "c"}, "/b/c"},
+	}
+	for _, tt := range tests {
+		t.Run(Join(tt.parts...), func(t *testing.T) {
+			if got := Join(tt.parts...); !reflect.DeepEqual(tt.want, got) {
+				t.Errorf("Join(%v) = %q; want %q", tt.parts, got, tt.want)
+			}
+		})
+	}
+}