ipn,cmd/tailscale,client/tailscale: add support for renaming TailFS shares

- Updates API to support renaming TailFS shares.
- Adds a CLI rename subcommand for renaming a share.
- Renames the CLI subcommand 'add' to 'set' to make it clear that
  this is an add or update.
- Adds a unit test for TailFS in ipnlocal

Updates tailscale/corp#16827

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

ipn/ipnlocal/tailfs_test.go

@@ -20,11 +20,11 @@ func TestNormalizeShareName(t *testing.T) {
 		},
 		{
 			name: "",
-			err:  errInvalidShareName,
+			err:  ErrInvalidShareName,
 		},
 		{
 			name: "generally good except for .",
-			err:  errInvalidShareName,
+			err:  ErrInvalidShareName,
 		},
 	}
 	for _, tt := range tests {