feature/drive: start factoring out Taildrive, add ts_omit_drive build tag

As of this commit (per the issue), the Taildrive code remains where it
was, but in new files that are protected by the new ts_omit_drive
build tag. Future commits will move it.

Updates #17058

Change-Id: Idf0a51db59e41ae8da6ea2b11d238aefc48b219e
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

cmd/tailscale/cli/cli.go

@@ -210,6 +210,7 @@ func noDupFlagify(c *ffcli.Command) {
 var fileCmd func() *ffcli.Command
 var sysPolicyCmd func() *ffcli.Command
 var maybeWebCmd func() *ffcli.Command
+var maybeDriveCmd func() *ffcli.Command
 
 func newRootCmd() *ffcli.Command {
 	rootfs := newFlagSet("tailscale")
@@ -262,7 +263,7 @@ change in the future.
 			updateCmd,
 			whoisCmd,
 			debugCmd(),
-			driveCmd,
+			nilOrCall(maybeDriveCmd),
 			idTokenCmd,
 			configureHostCmd(),
 			systrayCmd,