For TAB to CTRL just in Emacs, install Karabiner and configure TAB key under emacs mode.
Then for CTRL in all apps, goto OSX system preferences, keyboard, modifier keys, and then change it as needed.
For other operating systems, look into similar utilities.
Updated: some solutions may require using both solutions together. For example, you can re-assign TAB to be become Left Control key in Karabiner and then in system preferences enable/disable that Ctrl key's behavior as needed. This is one way to tweak laptop keyboards besides using any extended fn keys.