tailfs: replace webdavfs with reverse proxies

Instead of modeling remote WebDAV servers as actual
webdav.FS instances, we now just proxy traffic to them.
This not only simplifies the code, but it also allows
WebDAV locking to work correctly by making sure locks are
handled by the servers that need to (i.e. the ones actually
serving the files).

Updates tailscale/corp#16827

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

tailfs/tailfsimpl/shared/pathutil.go

@@ -40,3 +40,11 @@ func Join(parts ...string) string {
 func IsRoot(p string) bool {
 	return p == "" || p == sepString
 }
+
+// Base is like path.Base except that it returns "" for the root folder
+func Base(p string) string {
+	if IsRoot(p) {
+		return ""
+	}
+	return path.Base(p)
+}