Provide function_exists() checks for _s functions

Reading through `template-tags.php` I noticed that some of the _s functions declared had `function_exists()` checks, while some didn’t. I figured it’d be better to be consistent with this.

I went ahead and did this for `template-tags.php`. I also added the name of the function checked after each `endif`. (I found this pattern surrounding the `_s_setup()` declaration in `functions.php` and thought it’d be a good one to follow. I also kept the `add_action()` calls outside of the `function_exists()` checks, as I found with that declaration. Let me know if that should be switched.)

Should the rest of the _s functions be wrapped like this? (All found in `functions.php` and the files in `inc` as far as I can tell.) I can do it if it seems like a good plan.