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/remote_permissions_test.go

@@ -0,0 +1,61 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+package tailfs
+
+import (
+	"encoding/json"
+	"testing"
+)
+
+func TestPermissions(t *testing.T) {
+	tests := []struct {
+		perms []grant
+		share string
+		want  Permission
+	}{
+		{[]grant{
+			{Shares: []string{"*"}, Access: "ro"},
+			{Shares: []string{"a"}, Access: "rw"},
+		},
+			"a",
+			PermissionReadWrite,
+		},
+		{[]grant{
+			{Shares: []string{"*"}, Access: "ro"},
+			{Shares: []string{"a"}, Access: "rw"},
+		},
+			"b",
+			PermissionReadOnly,
+		},
+		{[]grant{
+			{Shares: []string{"a"}, Access: "rw"},
+		},
+			"c",
+			PermissionNone,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.share, func(t *testing.T) {
+			var rawPerms [][]byte
+			for _, perm := range tt.perms {
+				b, err := json.Marshal(perm)
+				if err != nil {
+					t.Fatal(err)
+				}
+				rawPerms = append(rawPerms, b)
+			}
+
+			p, err := ParsePermissions(rawPerms)
+			if err != nil {
+				t.Fatal(err)
+			}
+
+			got := p.For(tt.share)
+			if got != tt.want {
+				t.Errorf("got %v, want %v", got, tt.want)
+			}
+		})
+	}
+}