Make eligible functions static

The codebase is already very disciplined in making any function which
can be `static`, but there are a few that appear to have slipped through
the cracks.

test/unit/double_free.c

@@ -9,19 +9,20 @@ void fake_abort(const char *message) {
 	fake_abort_called = true;
 }
 
-void
+static void
 test_double_free_pre(void) {
 	safety_check_set_abort(&fake_abort);
 	fake_abort_called = false;
 }
 
-void
+static void
 test_double_free_post() {
 	expect_b_eq(fake_abort_called, true, "Double-free check didn't fire.");
 	safety_check_set_abort(NULL);
 }
 
-bool tcache_enabled() {
+static bool
+tcache_enabled() {
 	bool enabled;
 	size_t sz = sizeof(enabled);
 	assert_d_eq(