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>

ipn/ipnlocal/tailfs_test.go

@@ -0,0 +1,40 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+package ipnlocal
+
+import (
+	"fmt"
+	"testing"
+)
+
+func TestNormalizeShareName(t *testing.T) {
+	tests := []struct {
+		name string
+		want string
+		err  error
+	}{
+		{
+			name: "  (_this is A 5 nAme )_ ",
+			want: "(_this is a 5 name )_",
+		},
+		{
+			name: "",
+			err:  errInvalidShareName,
+		},
+		{
+			name: "generally good except for .",
+			err:  errInvalidShareName,
+		},
+	}
+	for _, tt := range tests {
+		t.Run(fmt.Sprintf("name %q", tt.name), func(t *testing.T) {
+			got, err := normalizeShareName(tt.name)
+			if tt.err != nil && err != tt.err {
+				t.Errorf("wanted error %v, got %v", tt.err, err)
+			} else if got != tt.want {
+				t.Errorf("wanted %q, got %q", tt.want, got)
+			}
+		})
+	}
+}