cmd/tailscale: make file cp send files via tailscaled localapi

So Taildrop sends work even if the local tailscaled is running in
netstack mode, as it often is on Synology, etc.

Updates #2179 (which is primarily about receiving, but both important)

Change-Id: I9bd1afdc8d25717e0ab6802c7cf2f5e0bd89a3b2
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

ipn/localapi/localapi.go

@@ -376,6 +376,25 @@ func (h *Handler) serveFileTargets(w http.ResponseWriter, r *http.Request) {
 	json.NewEncoder(w).Encode(fts)
 }
 
+// serveFilePut sends a file to another node.
+//
+// It's sometimes possible for clients to do this themselves, without
+// tailscaled, except in the case of tailscaled running in
+// userspace-networking ("netstack") mode, in which case tailscaled
+// needs to a do a netstack dial out.
+//
+// Instead, the CLI also goes through tailscaled so it doesn't need to be
+// aware of the network mode in use.
+//
+// macOS/iOS have always used this localapi method to simplify the GUI
+// clients.
+//
+// The Windows client currently (2021-11-30) uses the peerapi (/v0/put/)
+// directly, as the Windows GUI always runs in tun mode anyway.
+//
+// URL format:
+//
+//    * PUT /localapi/v0/file-put/:stableID/:escaped-filename
 func (h *Handler) serveFilePut(w http.ResponseWriter, r *http.Request) {
 	if !h.PermitWrite {
 		http.Error(w, "file access denied", http.StatusForbidden)