ipn/{ipnlocal/peerapi, localapi} initial taildrop resume api plumbing (#9798)

This change:
* adds a partial files peerAPI endpoint to get a list of partial files
* adds a helper function to extract the basename of a file
* updates the peer put peerAPI endpoint
* updates the file put localapi endpoint to allow resume functionality

Updates #14772

Signed-off-by: Rhea Ghosh <rhea@tailscale.com>

ipn/ipnlocal/peerapi_test.go

@@ -173,7 +173,7 @@ func TestHandlePeerAPI(t *testing.T) {
 			reqs:       []*http.Request{httptest.NewRequest("PUT", "/v0/put/foo", nil)},
 			checks: checks(
 				httpStatus(http.StatusForbidden),
-				bodyContains("Taildrop access denied"),
+				bodyContains("Taildrop disabled"),
 			),
 		},
 		{
@@ -280,7 +280,7 @@ func TestHandlePeerAPI(t *testing.T) {
 			reqs:       []*http.Request{httptest.NewRequest("PUT", "/v0/put/", nil)},
 			checks: checks(
 				httpStatus(400),
-				bodyContains("empty filename"),
+				bodyContains("invalid filename"),
 			),
 		},
 		{
@@ -290,7 +290,7 @@ func TestHandlePeerAPI(t *testing.T) {
 			reqs:       []*http.Request{httptest.NewRequest("PUT", "/v0/put/foo/bar", nil)},
 			checks: checks(
 				httpStatus(400),
-				bodyContains("directories not supported"),
+				bodyContains("invalid filename"),
 			),
 		},
 		{